In theory, the highest number usable is 65535 as this is the limit for the microsoft resource format.
Unfortunately, it appears that the gradient function does not support embedded rcclr calls. Because of this, you can't make a colorizable gradient. If you can add or replace a current color, (which last I heard wasn't supported), the binary format for such a gradient would be as follows:
(each <> part is one byte)
HC<rr><gg><bb><aa>C<rr><gg><bb><aa>
The H stands for horizontal gradient, (you'd use V if you wanted a vertical gradient)
C is a colorizable color. (as opposed to D for a normal color)
then the rr, gg, bb and aa parts are the desired colors and alpha in the gradient parts. (you'll want the alpha to be 255 (0xFF) most of the time)
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