You really should not use char maths in production for this kind of problems. Bad "colors" are an arbitrary human-defined list, so you should clearly define it as a const/var for later corrections. If they suddenly start using z or Ω or 🍆 the code should still work.

As already mentioned, the actual solution to the problem includes two 'ceiling' calls, otherwise the result may get smaller than necessary.
Writing a solution that's wrong and needed just to pass the test is as bad of an exercise as it can possibly be.

I understand the irony of the situation in regards to coding as a real life job, but this is not the place.

Rust version reported missing.