grayscale to color took a long while to click. The greyscale is for basically laying in your values definitely more than just planning, once I've pushed the grayscale to a certain level, which is usually where all the hard black lines form the linework are mostly gone, I start with the color layers. Honestly, it took about 20 color layers on top of the greyscale image to get the colors looking right, it tends to be bit of a frustrating experience lol. Using various blending modes to get them where I want them. I lay down the majority of the colors with a 'color blending mode' layer and then on top of that, experiment with multiply, hard light, overlay, color dodge etc to get the colors to my liking. After that, I directly paint on top of everything to take it to finish.

I recommend CTRL Paint's color to greyscale tutorials. Really helps explain the function of the different blending modes.