I know that in the 2009 MUTCD the FHWA made a big deal about sign retroreflectivity, setting minimum values for most color combinations, with the notable exceptions of white on blue and white on brown. However, these color combinations have since been tested and retroreflectivity values for them will likely be included in the next edition of the MUTCD. However, there are a few other combinations out there that still haven't been tested. Here are some examples:
- Black on Fluorescent Pink (Incident Management)
- White on Purple (Electronic Toll Collection)
- Yellow on Blue (County Shields)
I wonder if there will ever be testing done on these combinations.
A new MUTCD is in the works and proposed changes are open to comment, haven't checked the document but I wonder if these changes you suggest are in there.