mode 7 background

It means its going to take a little more effort than changing a color in mspaint. If the disassembly doesn't show it then it will take some digging into to figure out the code, requiring a good working knowledge of the assembly code. There are a number of good sites on the subject, after that you will want to start looking over the code in the disassembly for where the color selection might logically take place (like during take off), then start looking it over byte by byte with your assembly guide and translate it, looking for a value that might be that background color. It will be tedius and likely hard to follow, but you will learn a lot and it will feel good to find it.