I have a strong memory that AFL - american fuzzy lop the binary fuzzer had a feature similar to what this was doing based on the highlighted portions and screenshots. It wasn't the AFL status screen, it was (may have been a) third party app, and it would color code parts of the input files based on the outputs or whatever from afl's processing.

For example, there was a color key that explained that say, purple meant "magic bytes", like "0x4a46494600" for JFIF0, and if any part of the input file caused errors it meant it was probably a checksum and needed to be "fixed" so afl could properly fuzz all the functions in the source code.

I'm not super in to fuzzing or that realm anymore, so i doubt i could describe it better than i did, here. I clicked through to see if someone have leveraged the AFL stuff for use in another tool, which would be cool.

edit: i think it was afl-analyze - i had a go at the source code for aflplusplus:

> A nifty utility that grabs an input file and takes a stab at explaining its structure by observing how changes to it affect the execution path.

> Another tool in AFL++ is the afl-analyze tool. It takes an input file, attempts to sequentially flip bytes and observes the behavior of the tested program. It then color-codes the input based on which sections appear to be critical and which are not; while not bulletproof, it can often offer quick insights into complex file formats.