Remove Colours array in -print-changed=dot-cfg

Summary:
The Colours array is apparently the source of TSAN errors. It is
unnecessary and was there to ease readability of the code. Remove it to
clean up the TSAN errors.

Author: Jamie Schmeiser <schmeise@ca.ibm.com>
Reviewed By: aeubanks (Arthur Eubanks)
Differential Revision: https://reviews.llvm.org/D115175