There are a few that may be able to help. Some artists prefer the more direct approach of using a psychic-type, such as a member of the abra or ralts lines, to use their telepathic abilities to impose images of the colors their trainer wishes to capture on their mind. However, this is also a rather invasive approach, so not that many artists prefer to use it outside of more complicated pieces. (Additionally, excessive use of an abra, kadabra, or alakazam’s abilities may induce rather nasty headaches due to overexposure to psi waves.)
For that reason, most artists in your situation prefer a more synesthetic approach. Rather than forcing themselves to see the colors they need, they prefer using sight or sound to compensate. For example, they may catch a chingling or chimecho (or any pokémon with a very distinctive call) and train these pokémon to emit very specific tone-and-syllable combinations that match different shades of a certain color. A high-pitched “chime” may indicate a lighter shade of red, whereas a lower-pitched “echo” may refer to a dark green. This pokémon can then be asked to survey a model for any color that may present a challenge for their trainers, then indicate that object’s general color and shade with the corresponding tone they were taught to use. They can then guide their partners through the mixing of these colors by observing the artist’s paints and emitting tones until the pokémon’s sound matches the one they used while observing the model exactly.
The olfactory technique works in a similar manner, wherein an artist uses a pokémon capable of controlling their scent (such as pokémon that can use Sweet Scent, members of the budew family, or—in rare cases involving more eccentric artists—stunky or skuntank) to match the scent of a model with the scent of the paint an artist uses. However, because odors can sometimes take time to dissipate, it’s a less popular technique than the auditory one.
Best of luck, anonymous!
The Pokédex According to Bill 