Palettes are most likely stick together in rom file in a single common table. So you already know the search area.
To find out which byte exactly to change, open ppu viewer in fceux emulator, hover cursor over 2nd line of colors (those are for sprites). To make sure it's the right color, open hex editor - ppu memory - go to $3F10 line and change bytes here manually (sometimes it will change back right away, try to slow down emulation to notice changes on the screen).
Then find the same byte in your table. There can be several same bytes nearby, try them all, one by one.
Then reload the level or load a proper savestate to update colors when code will read them.
For more precice search you can try to find 2-4th bytes of colors together from a set of colors for a sprite (there are 4 sets, each one has 4 colors). Also use code/data logger to see bytes which were read by the game code while it's working.