I'm fairly new to Blender so there might be an obvious answer to this, but I need to be able to create a texture based off of multiple materials, sort of. I'm making low poly game assets, and have just been assigning materials to faces to get them "textured", then exporting them as .fbx files and using them in Unity. However, this creates WAY too many materials in Unity. I'd like to just use a uv map, but whenever I try painting, the colors run into each other a little bit at the edges. Is there any way to translate multiple materials into one .png?
Baking is the way to go. If you're in blender internal, baking is very straightforward. If using Cycles it is a little more complex but can give great results. Create an image for each map that you need, open that image and select the node in the Cycles node editor, then bake the map you need in the bake panel. It can combine the diffuse map from multiple UV projections, procedural materials etc.