The Scott Specialized catalog shows two color varieties of the normal blue color for R54, the 50 cent Conveyance stamp: ultramarine and powder blue. I don't see powder blue listed for any of the other blue first issue revenues, so perhaps it is unique to R54.
In previous Scott Specialized catalogs, they also listed a color called steel blue. I don't know why this color is no longer listed.
I have attached some stamp images below of what I believe to be representative of these color names, along with a new one that I would call milky blue.

First is the basic blue color.

This is powder blue.

This is bright violet blue, or what Scott calls ultramarine.

This is dull violet blue.

Steel blue (purchased as such from Eric Jackson 10 years ago).

Milky blue.
Comments appreciated. I don't have certificates for any of these. Just looking for a concensus of opinion.
Thanks,
Jim