The aptly-named Pokemon Red/Blue Color Hack gives you what it says on the tin: fully colorized versions of Pokemon Red and Blue.
Some may be aware that, in fact, Pokemon Red and Blue did have some color when played on a Super Gameboy. However, the Super Gameboy's colorization capability was extremely limited, being unable to assign separate colors to the background and sprite layers, among other limitations. This hack, targeting the Gameboy Color, does not have these limitations. As you can see in the screenshots, this colorization is akin to the gen 2 games.
This hack also optionally replaces the pokemon sprites with gen 2 equivalents, since some of the gen 1 spritework was... questionable. The battle UI has also been ported from the gen 2 games. However, the patch is available in a "vanilla" variant for those who prefer the original spritework and battle UI.
Previous versions of this hack were plagued by graphical corruption bugs. After much headbanging and many headaches, these bugs have been fixed. It has been tested extensively on a real gameboy, and the bugs have not resurfaced. However, it must be noted that the VBA emulator has issues with this hack. VBA-M works, though, as do the more accurate emulators such as BGB.RHDN Project PageRelevant Link