As far as I know, there's only one person that managed to accomplish that. Diesieben from https://www.minecraftforge.net/forum/ replied to an older thread where someone else asked how to create colored lights, and you can find his answer here. As he said it will take a long time and a lot of java knowledge.
I think it's a waste of time to even look into it.
You'd have to separate light into 3 components, one each for red, green, and blue, which would require extensive modifications to vanilla code, including optimizations to offset the impact of having to handle 3 separate channels instead of one (or two, for block light and sky light; if only block light is modified the total number of channels is 4). Luckily, the rendering code already works with separate R,G,B values so you wouldn't have to modify it other than to assign a different light component to each color instead of handling them all at once, and have the "brightness" instead set the color (that is, the rendering code separately sets the color and brightness of each vertex, where color is used to color the block, like e.g. grass, and brightness is used to assign a brightness value based on a light map, which currently determines the color of light. A single lightmap could still be used with colored light, which would affect how "white" appears and tint other colors, similar to how stained clay is tinted red; if pure colors are desired you could override the light map, which is internally generated).
Also, IMO, Forge is very poorly suited for this sort of thing as it makes you use highly advanced things like ASM/working with bytecode to modify vanilla code (sort of like being forced to use assembly language instead of C++ to make a mod for a C++ based program), which is why I do not use it, though of course this means my mods are incompatible with just about everything else; however, their performance (resource/memory usage in particular) is vastly superior to Forge mods since I can directly work with the base code. Of course, even a Forge mod would probably not work with many other mods that affect the rendering engine (so no Optifine).
In either case this is something that should be done as part of a complete overhaul, not just so one block can have colored light, due to the amount of work that would be required.