4zb4, awesome!
You can use
GIMP to open, edit, then "Export As... -> ZSoft PCX image". I recommend taking the original explosions in
Symbol.pcx (just click Raw to download) and simply rearranging the palette such that the yellows are #192-207 and the greys are 208-223. You can do that by opening the file in GIMP and going to "Colors->Map->Rearrange Color Map..." and dragging them around. You should then go through all the original yellow and greys in that file that aren't explosions to make sure they're not in those high color ranges. For an explanation of why you might want to rearrange a color map, take a look at
this explanations of colors in IVAN.
Alternatively, you could use
ImageMagick to write a few commands to recolor the explosions so that they use the colors in the special bands or to rearrange the palette. That's somewhat more advanced, but you can find examples of the commands to accomplish most anything in ImageMagick with a bit of Googling.
This will solve
this note about M-colored explosions in the old developer notes on graphics, which is pretty nice to cross off.