A couple things I do at this stage:
- Be loose and messy
- Use a large Airbrush tool to cover large areas fast
- Select large areas with the Lasso tool and apply transparent washes of color
- Use Hue Saturation, Levels, Color Balance to the entire image or parts of the image to quickly try out different looks.
All these things help to achieve unique colors in the final illustration that wouldn't normally happen if you are working slowly and methodically. Sometimes unusual colors appear in areas of the illustration that I'd never think to put there, but they look good, create some interest and make the palette richer.
Going to Final
When I'm happy with the color rough I usually paste the original line sketch on top of the color rough and apply "Multiply" to the line sketch layer because the lines of the drawing are usually lost or covered up now. I then start painting directly on top in Photoshop. This way it is more likely that "happy accidents" that occurred in the color rough phase will make it into the final illustration.
I used to not paint directly on top of my color rough and start from a pristine version of my line sketch instead. If you do it this way, you can use your color rough as your artist palette by keeping it open as a separate file and clicking on it with the Eyedropper tool just as you would go to your paint palette if painting with real paint.
PHOTOSHOP TIP: Get out of the habit of using the tool buttons to select your tools. You can quickly toggle to the Eyedropper tool holding the Option key or setting your Wacom pen button to be the "Option" key. After getting used to the shortcut for the Eyedropper tool it is so fast and seamless to change brush colors. I only use the color picker when the color I picked up is not quite right. I then use the color picker to fine tune the color from the color that was selected from the color rough.
But something I do use layers for is when I am more in the final stages of the illustration and I want to try something on top of what I already have without messing up what I already have. This is useful especially for drawing lines on top of the illustration. I am jealous of people who can draw lines confidently and not mess up. That isn't me. I am much more confident mushing colors around with a brush. So doing critical lines on a separate layer allows me to go back and make it right if I have to.