Pokemon's Pokedex entries have always been a point of contention among fans as they tend to change between mediums, leading to confusion about what is canon and what isn't. While this tends to be the case with entries for specific Pokemon, other entries have mentioned Pokemon biology as a whole. In particular, Season 1 Episode 42 "Showdown at Dark City" has hinted that most Pokemon may in fact be colorblind.
In the episode, Ash, Brock, and Misty come to a Western-style town called Dark City where two factions were fighting against each other in a bid to try and become official Pokemon gyms. The Yas and Kaz groups have been causing all sorts of trouble in the town to try and get rid of each other before the Pokemon League Inspector arrived to choose a new official gym. The Yas Gym featured a Scyther, while the Kaz Gym featured an Electabuzz. When using his Pokedex on these Pokemon, Ash is told, "Scyther and Electabuzz are both rare Pokemon. They are able to distinguish colors and become enraged by the color red.”
This could just be the anime making a reference to the fact that both Pokemon were exclusive to the Red version of the original game, but if taken at face value, it implies that in general, most Pokemon are likely colorblind. This is backed up by Spearow's entry earlier in the series, which claimed that it was colorblind. However, it could also be the exception.
If Pokemon are based on animals, then it would make sense that they would take on aspects of their real-life counterparts. Birds can see in an ultraviolet range, meaning they actually see more colors than humans. It could be that bird Pokemon can also see in this spectrum and Spearow is the exception to this. Zubats are blind and use ultrasonic waves to gets around via echolocation, just like real-world bats do. Luxray is said to be able to see through walls, either via X-ray vision or by using Gamma rays. It would make sense for Pokemon like Seel and Dewgong to be colorblind because seals and whales lack cones, which are the portion of the eye that sense colors.
The biology of Pokemon is something that isn't really talked about in the anime series. Usually, the Pokedex talks more about the nature of the Pokemon and its notable qualities, but viewers are rarely told about the science. This has led many people to completely ignore the Pokedex, not helped by the fact that the entries can sometimes be inconsistent.
This also brings up the question of how Pokemon that aren't based on animals see. Would they be colorblind as the standard, or would some be able to see in different spectrums? Plants can't see, so what about Pokemon based on flowers and trees? This is something that the series could expand on more that fans would actually want to know more about.