How often do you have these hallucinations? Also, have you ever painted anything before?
I used the word hallucinations, because I couldn't think of a better one. It's more an urge to do something that is so strong that I can almost feel myself doing it. I also have a very strong visual imagination, so I can imagine the end result down to fine details of colour and composition. Yes, I have been an amateur painter and sculptor for many years and have sold quite a few pieces.
I know exactly what you're talking about! At least, I think I know.

A few months back I had a series of dreams where the urge to tattoo a small, curved fine-line fern leaf or a single feather just behind my right ear was so strong that I almost gave into the impulse. While dreaming it just made so much sense and felt so right...I was absolutely decided I was going to get it.
However, every time I woke up and the 'brain brakes' went online again, the impulse faded significantly. It was just an impulse -- a very strong urge -- to do something that probably isn't really a good idea. I thought it through and decided, probably not. It's probably not a good idea.
That was an urge for something that would have been permanent on my skin so I had to give it plenty of thought. Painting what you saw in your dream could be fun, if I were you I would give into the impulse and do it.