Create Rounded Rectangle Texture2D

I am building a game with dialog boxes, and I would like to be able to procedurally generate textures for the boxes, in a very similar style to that of the Final Fantasy Series (think like Final Fantasy VII). Here is a code snippet of what I have so far:

public class DialogBox
{
    public Rectangle BoxArea { get; set; }
    public List<Color> BoxColors { get; set; }
    public List<Color> BorderColors { get; set; }
    public int BorderThickness { get; set; }
    public int BorderRadius { get; set; }
    private Texture2D texture;

    public void CreateBackdrop(ref GraphicsDevice graphics)
    {
        texture = new Texture2D(graphics,
                                BoxArea.Width,
                                BoxArea.Height,
                                true,
                                SurfaceFormat.Color);
        Color[] color = new Color[texture.Width * texture.Height];

        for(int x = 0; x < texture.Width; x++)
        {
            for(int y = 0; y < texture.Height; y++)
            {
                switch(BoxColors.Count)
                {
                    case 4:
                        Color leftColor = Color.Lerp(BoxColor[0], BoxColor[1], (y / (texture.Height - 1)));
                        Color rightColor = Color.Lerp(BoxColor[2], BoxColor[3], (y / (texture.Height - 1)));
                        color[x + y * texture.Width] = Color.Lerp(leftColor,
                                                                  RightColor,
                                                                  (x / (texture.Width - 1)));
                        break;
                    case 2:
                        color[x + y * texture.Width] = Color.Lerp(BoxColors[0],
                                                                  BoxColors[1],
                                                                  (x / (texture.Width - 1)));
                        break;
                    default:
                        color[x + y * texture.Width];
                        break;
                 }
            }
        }
        texture.SetData<Color>(color);
    }
}

What I am looking for is the following:

4-way gradient color (implemented)
Rounded Rectangle
Border with gradient

Any help would be greatly appreciated.

I figured out the rectangle via this question.

I figured out what I was having trouble with; calculating the corner arcs. What I figured out was that one could use distance from a point to understand the way to draw the border, rather than using mathematics to calculate the points within a circle. Below I have posted the code to generate the rounded rectangle.

public Texture2D CreateRoundedRectangleTexture(GraphicsDevice graphics, int width, int height, int borderThickness, int borderRadius, int borderShadow, List<Color> backgroundColors, List<Color> borderColors, float initialShadowIntensity, float finalShadowIntensity)
{
    if (backgroundColors == null || backgroundColors.Count == 0) throw new ArgumentException("Must define at least one background color (up to four).");
    if (borderColors == null || borderColors.Count == 0) throw new ArgumentException("Must define at least one border color (up to three).");
    if (borderRadius < 1) throw new ArgumentException("Must define a border radius (rounds off edges).");
    if (borderThickness < 1) throw new ArgumentException("Must define border thikness.");
    if (borderThickness + borderRadius > height / 2 || borderThickness + borderRadius > width / 2) throw new ArgumentException("Border will be too thick and/or rounded to fit on the texture.");
    if (borderShadow > borderRadius) throw new ArgumentException("Border shadow must be lesser in magnitude than the border radius (suggeted: shadow <= 0.25 * radius).");

    Texture2D texture = new Texture2D(graphics, width, height, false, SurfaceFormat.Color);
    Color[] color = new Color[width * height];

    for (int x = 0; x < texture.Width; x++)
    {
        for (int y = 0; y < texture.Height; y++)
        {
            switch (backgroundColors.Count)
            {
                case 4:
                    Color leftColor0 = Color.Lerp(backgroundColors[0], backgroundColors[1], ((float)y / (width - 1)));
                    Color rightColor0 = Color.Lerp(backgroundColors[2], backgroundColors[3], ((float)y / (height - 1)));
                    color[x + width * y] = Color.Lerp(leftColor0, rightColor0, ((float)x / (width - 1)));
                    break;
                case 3:
                    Color leftColor1 = Color.Lerp(backgroundColors[0], backgroundColors[1], ((float)y / (width - 1)));
                    Color rightColor1 = Color.Lerp(backgroundColors[1], backgroundColors[2], ((float)y / (height - 1)));
                    color[x + width * y] = Color.Lerp(leftColor1, rightColor1, ((float)x / (width - 1)));
                    break;
                case 2:
                    color[x + width * y] = Color.Lerp(backgroundColors[0], backgroundColors[1], ((float)x / (width - 1)));
                    break;
                default:
                    color[x + width * y] = backgroundColors[0];
                    break;
            }

            color[x + width * y] = ColorBorder(x, y, width, height, borderThickness, borderRadius, borderShadow, color[x + width * y], borderColors, initialShadowIntensity, finalShadowIntensity);
        }
    }

    texture.SetData<Color>(color);
    return texture;
}

private Color ColorBorder(int x, int y, int width, int height, int borderThickness, int borderRadius, int borderShadow, Color initialColor, List<Color> borderColors, float initialShadowIntensity, float finalShadowIntensity)
{
    Rectangle internalRectangle = new Rectangle((borderThickness + borderRadius), (borderThickness + borderRadius), width - 2 * (borderThickness + borderRadius), height - 2 * (borderThickness + borderRadius));

    if (internalRectangle.Contains(x, y)) return initialColor;

    Vector2 origin = Vector2.Zero;
    Vector2 point = new Vector2(x, y);

    if (x < borderThickness + borderRadius)
    {
        if (y < borderRadius + borderThickness)
            origin = new Vector2(borderRadius + borderThickness, borderRadius + borderThickness);
        else if (y > height - (borderRadius + borderThickness))
            origin = new Vector2(borderRadius + borderThickness, height - (borderRadius + borderThickness));
        else
            origin = new Vector2(borderRadius + borderThickness, y);
    }
    else if (x > width - (borderRadius + borderThickness))
    {
        if (y < borderRadius + borderThickness)
            origin = new Vector2(width - (borderRadius + borderThickness), borderRadius + borderThickness);
        else if (y > height - (borderRadius + borderThickness))
            origin = new Vector2(width - (borderRadius + borderThickness), height - (borderRadius + borderThickness));
        else
            origin = new Vector2(width - (borderRadius + borderThickness), y);
    }
    else
    {
        if (y < borderRadius + borderThickness)
            origin = new Vector2(x, borderRadius + borderThickness);
        else if (y > height - (borderRadius + borderThickness))
            origin = new Vector2(x, height - (borderRadius + borderThickness));
    }

    if (!origin.Equals(Vector2.Zero))
    {
        float distance = Vector2.Distance(point, origin);

        if (distance > borderRadius + borderThickness + 1)
        {
            return Color.Transparent;
        }
        else if (distance > borderRadius + 1)
        {
            if (borderColors.Count > 2)
            {
                float modNum = distance - borderRadius;

                if (modNum < borderThickness / 2)
                {
                    return Color.Lerp(borderColors[2], borderColors[1], (float)((modNum) / (borderThickness / 2.0)));
                }
                else
                {
                    return Color.Lerp(borderColors[1], borderColors[0], (float)((modNum - (borderThickness / 2.0)) / (borderThickness / 2.0)));
                }
            }


            if (borderColors.Count > 0)
                return borderColors[0];
        }
        else if (distance > borderRadius - borderShadow + 1)
        {
            float mod = (distance - (borderRadius - borderShadow)) / borderShadow;
            float shadowDiff = initialShadowIntensity - finalShadowIntensity;
            return DarkenColor(initialColor, ((shadowDiff * mod) + finalShadowIntensity));
        }
    }

    return initialColor;
}

private Color DarkenColor(Color color, float shadowIntensity)
{
    return Color.Lerp(color, Color.Black, shadowIntensity);
}

With all of the code above, this is the result: