To change the color of the dark squares, add this to the end of the URL:

&ds_color=(d,d,d)

The d’s represent the red, green, and blue components of the color you’d like. Replace the d’s with numbers between 0 and 255. (Don’t use any spaces.) Here are some sample colors:

• White: (255,255,255)
• Black: (0,0,0)
• Red: (255,0,0)
• Green: (0,255,0)
• Blue: (0,0,255)
• Cyan: (0,255,255)
• Magenta: (255,0,255)
• Yellow: (255,255,0)

To change the color of the light squares, add this:

&ls_color=(d,d,d)

Here’s a board with different choices for both the dark and light squares:

The complete URL for the image above is:

http://www.eddins.net/steve/chess/ChessImager/ChessImager.php?
fen=rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR/&ds_color=(70,109,186)&ls_color=(255,220,175)

(You need to put that all on one line, with no spaces.)