LSD
Most of your requirements could be filled with LSD or a family-adjacent "research chemical" which are very similar in molecular structure.
The paint drugs have similar properties(appearance and consistency) to paint, enough that they can be painted on an actual canvas.
Should be possible with a skilled chemist to make a paint that can contain LSD without it decomposing. Since dosages of LSD for recreational use are usually counted in hundreds of micrograms, the paint could easily retain psychoactive effects while still being over 99.9% simple paint with only a very small fraction of the contents being "drugs".
They are not meant to be ingested(possible but dangerous) but to be used as paint.
Their psychoactive properties are delivered through smell and skin contact. The longer you interact with them or the more you use, the more potent the effects become.
When stored or in a 'still' state, their effects are kept at a minimum. Only when the substances are in an agitated state(such as being painted on something) would the effects be maximized.
LSD is fastest absorbed through any mucus membrane. Recreational users typically let the dose be absorbed in their mouth or eyes (or they simply eat it). That said, in high enough dosages, it can be absorbed directly through skin contact. With a high-enough concentration of LSD in the paint, you could have it so that artists who get it on their hands slowly experience the effects as it is absorbed through their pores, and this would also mean that if they accidentally ate it, the dosage would be extremely high and debilitating.
Unfortunately for you, LSD is not really directly "dangerous" even in extremely large doses. Nobody has ever overdosed on it, however scientists have identified a theoretical lethal dosage of 100mg in humans through animal testing (such a dose is roughly 1000x as much as a normal dose).
Getting the drug to work with smell as a transmission path too will probably be difficult though. You don't need a lot of LSD to experience the effects, which works in favor of somehow transferring it through the vapors, and perhaps your best bet would be making these paints into aerosol spray cans so that tiny droplets get breathed in when being used.
They can be any color. Each color can have a different psychoactive effect.
This is the most difficult part of your question I think. Drugs are very hard to engineer, in the sense that even modern medicine can't exactly explain how they work in many cases, and the effects they cause in different users are also unpredictable due to the unique biochemistry and neurology of every single user. Most are discovered on accident (LSD included) and setting out to create a drug that reliably induces a specific effect in people is something that pharma companies spend literal billions and billions of dollars on.
Creating or even randomly discovering a single drug is totally plausible, but creating/discovering a whole family of similar drugs yet that have very different effects like your paint colors stretches my credulity a bit.
Summary
Put LSD into aerosol spray-paint cans, and have the artist work without a mask in a poorly ventilated area. Breathing in the droplets will thoroughly dose them over time, and in terms of LSD's effects, it could definitely drive someone to pursue some creative pursuit to the point of ignoring external factors such as personal safety.