Generating Color Gradient

I want to display to my user how many (percent wise) of their 开发者_JAVA技巧forms are compliant with the new standard. The way I want to let them know visually is the percent amount will be colored. It will be 0xFF0000 (pure red) for 0% and 0x00FF00 (pure green) at 100%. What is the best way to calculate the color for each step along the way?

Colour space conversion (as suggested by Tony) will give you the best results. If however this is beyond the scope of what you are looking for, I suggest a simple algorithm that gets you yellow (0xFFFF00) for 50 %:

For values up to 50 % Start with 0xFF0000.
Add 0xFF * Percentage / 50 to the green component.

For values above 50 % Start with 0xFFFF00.
Subtract 0xFF * Percentage / 50 from the red component.

The results look good enough for my customers ;-)

You don't need to calculate it yourself - try using a LinearGradient brush. (msdn)

LinearGradientBrush linGrBrush = new LinearGradientBrush(
   new Point(0, 10),
   new Point(200, 10),
   Color.FromArgb(255, 255, 0, 0),   // Opaque red
   Color.FromArgb(255, 0, 0, 255));  // Opaque blue

Pen pen = new Pen(linGrBrush);

e.Graphics.DrawLine(pen, 0, 10, 200, 10);
e.Graphics.FillEllipse(linGrBrush, 0, 30, 200, 100);
e.Graphics.FillRectangle(linGrBrush, 0, 155, 500, 30);