A nice post by raasiel on dZone about writing a simple Custom MarkupExtension.



He creates a MarkupExtension you can use to automatically create a lighter and darker shade of a color for MouseOver effects. Interesting, although he doesn't post the code to actually create a darker or lighter color.
After reading A good introduction to the different color models that exist, I decided that the easiest a way to achieve the lighter and darker colors would be to use the HSL colormodel and adjust the lightness. The selected color would have a L of 50%, the darker would have a bit less, the lighter a bit more.
Now, since WPF uses RGB by default, we have to do some conversion. These code samples give a conversion algorithm, , and converting them to C# isn't really hard:

 ColorHSL RGBtoHSL(Color colorRGB)
{
    ColorHSL hsl = new ColorHSL();

    float r, g, b, h, s, l; //this function works with floats between 0 and 1 
    r = colorRGB.R / 256.0f;
    g = colorRGB.G / 256.0f;
    b = colorRGB.B / 256.0f;

    float maxColor = Math.Max(r, Math.Max(g, b));
    float minColor = Math.Min(r, Math.Min(g, b));

    
    //R == G == B, so it's a shade of gray
    if ( r == g && r == b )
    {
        h = 0.0f; //it doesn't matter what value it has       
        s = 0.0f;
        l = r; //doesn't matter if you pick r, g, or b   
    }
    else
    {   
        l = (minColor + maxColor) / 2;     
        
        if (l < 0.5) s = (maxColor - minColor) / (maxColor + minColor);
        else s = (maxColor - minColor) / (2.0f - maxColor - minColor);
        
        if(r == maxColor) h = (g - b) / (maxColor - minColor);
        else if(g == maxColor) h = 2.0f + (b - r) / (maxColor - minColor);
        else h = 4.0f + (r - g) / (maxColor - minColor);
        
        h /= 6; //to bring it to a number between 0 and 1
        if(h < 0) h ++;
    }

    hsl.H = (short)(h * 255);
    hsl.L = (short)(l * 255);
    hsl.S = (short)(s * 255);

    return hsl;
}

So, if we have the color as HSL, we can simply change the lightness and get the light and dark variant:


ColorHSL lightColor = (short)(normalColor.H * 1.5);
ColorHSL darkColor = (short)(darkColor.H * 0.5);


After this we have to convert back to RGB (mind you, this code is needs some improvements):

Color HSLtoRGB(ColorHSL colorHSL)
{
    double r, g, b, h, s, l; //this function works with floats between 0 and 1
    double temp1, temp2, tempr, tempg, tempb;
    h = colorHSL.H / 256.0;
    s = colorHSL.S / 256.0;
    l = colorHSL.L / 256.0;

    //If saturation is 0, the color is a shade of gray
    if (s == 0)
    {
        r = g = b = l;
    }
    else
    {
        //Set the temporary values      
        if (l < 0.5)
        {
            temp2 = l * (1 + s);
        }
        else
        {
            temp2 = (l + s) - (l * s);
        }

        temp1 = 2 * l - temp2;
        tempr = h + 1.0 / 3.0;

        if (tempr > 1)
        {
            tempr--;
        }
        
        tempg = h;
        tempb = h - 1.0 / 3.0;
        if (tempb < 0) 
            tempb++;

        //Red     
        if (tempr < 1.0 / 6.0) r = temp1 + (temp2 - temp1) * 6.0 * tempr;
        else if (tempr < 0.5) r = temp2;
        else if (tempr < 2.0 / 3.0) r = temp1 + (temp2 - temp1) * ((2.0 / 3.0) - tempr) * 6.0;
        else r = temp1;

        //Green       
        if (tempg < 1.0 / 6.0) g = temp1 + (temp2 - temp1) * 6.0 * tempg;
        else if (tempg < 0.5) g = temp2;
        else if (tempg < 2.0 / 3.0) g = temp1 + (temp2 - temp1) * ((2.0 / 3.0) - tempg) * 6.0;
        else g = temp1;

        //Blue    
        if (tempb < 1.0 / 6.0) b = temp1 + (temp2 - temp1) * 6.0 * tempb;
        else if (tempb < 0.5) b = temp2;
        else if (tempb < 2.0 / 3.0) b = temp1 + (temp2 - temp1) * ((2.0 / 3.0) - tempb) * 6.0;
        else b = temp1;
    }

    return Color.FromRgb(Convert.ToByte(r*255), Convert.ToByte(g*255), Convert.ToByte(b*255));
} 

If you use this code to calculate the colors you get this:



With this code you can complete your custom Markup Extension.