You could print a special edition of the manual in color and sell it separately, for the big fans. I'd suggest selling it with a copy of the game, but most of the biggest fans probably pre-ordered a long time ago, so they'd have to pay for the game again.
Could always do both though. I don't know what your minimum orders have to be for numbers of manuals, so it might not be worth it if only a few dozen (or less) people want to buy a color manual...