If you can do it, so that I can really change the text colours for each screen, I can wait. It would also be good for other modders as well. That's why I especially asked for this.
Is this a request? We like to help people learn how to do it here.
Assuming there is a suitable palette already loaded, what you would need to look for is data written what is known as the "attribute table". It tells the game which palette to use for each 16x16 pixel reqion of the screen.
FCEUX' debugger could help that. You would need to play the game and before the text is printed, set a Breakpoint on PPU Writes to $23C0-23FF in VRAM.
So that when the game tries to write to the Attribute Table, you can see what data that is.
The problem I could see is that, since the level is already the same colors, the game might not even bother updating the AT.
Oh wait, you're talking about changing the text in SMB1.
Well, I believe the above still applies.
We have a disassembly for that game so someone could easily check if the AT stuff is feasible or if that would need extra coding added (which I hear SMB1 has almost no free space).