I have come to the conclusion that Red and Blue didn't have Pokedex colour.
Because it was monochromatic back then. Yeah. xD
It seems that Pokedex Colour was introduced in Gen III, looking through Serebii's pages through the generations. And what was introduced? DETAILED COLOUR!! :D :D :D :D
So yeah, since Missingno. only had an appearance in Red, Blue and Yellow, where there was no Pokedex colour, Missigno. is colourless. :3
Hope I helped. :)
Source: Serebii and Bulbapedia