Custom FeedCount. Acording to which rule FeedBurner converts colors?

Question

I am traying to prepare own dynamicly generated FeedCount for my own subscription service. I think this will be a good idea to support convention for setting colors via URL which is well known from FeedBurner,

http://feeds.feedburner.com/~fc/Ekundelekpl?bg=99CCFF&fg=000000&anim=0

this URLs produce such image.

fg is means a text color, bg means here a dark blue color. However light blue (ccffff) is here generated according to some rule.

Is this a some kind popular convert color convention, which is well known in webdeveloper/designers world?

I checked few other pairs of color, but I count figure out what the rule is. I don't want to hardcode all colors supported by FeedBurner.

[EDIT]

There is 216 available colors. Here is twelve pairs which I have extracted:

000000,7e7e7e
000033,8a8a8a
000066,909090
000099,9999CC
0000CC,9999FF
0000FF,9999FF

006600,909090
006633,99CC99
006666,A2A2A2
006699,99CCCC
0066CC,99CCFF
0066FF,99CCFF

Answer 1

I did it in another way.

I decided to use feedburner as template. I download proper image and override parts which interest me.

https://github.com/noisy/eGazeciarzFeedCount