If you open the PPU viewer and change weapons, you'll see that the first sprite palette changes colors, so that must be which palette Mega Man uses. If you highlight the colors, you will see that Mega Man's default colors are "0F 2C 11" and Bubble Lead is "0F 30 00". Simply open the rom in a hex editor and search for "0F 30 00". It just so happens that there is only one single instance of that string of bytes in the whole rom, so it must be the Bubble Lead color. It is! Change (don't insert new bytes, you might have to press the insert key, depending on the hex editor) those bytes and then save it.