One of the first things I had to do during my apprenticeship after doing many quick studies on ProCreate was to learn how to paint in watercolor. I had to learn realism first in both color and black and grey, and my subjects had to be botanicals due to their forgiving nature to draw.
While tattooing in and of itself is an entirely different medium, the way that watercolor reacted on paper had similarities to how ink would react on human skin.
I had to become familiar with the way that colors had to be built to be saturated with the watercolor. Its biggest difference to tattooing is its forgiving nature. Once I was able to gain some proficiency in watercolor, I was able to move on to studying Sumi-e, or Japanese Ink Painting.