Well @Marco Bucci 's 10 minutes to better painting is my go-to recommendation there. You don't learn "colouring" by itself, it's a system that works together with other elements that forms a image.

You might start by doing a color wheel and learning the primaries, secodary, and tertiary colors. Learn the definitions of hue, value, and saturation. Then do some color studies from masterful paintings, just trying to copy the colors. Most of the time when a color looks off, it's the value. You can usually get away with incorrect color if the value is correct Janes Gurney's Color and Light is a great book. Nathan Fowke's courses on Schoolism are also great

I support Sonja Müller's suggestion to do color studies. In addition, pick up some color theory. Some suggestions are http://www.huevaluechroma.com, http://handprint.com/HP/WCL/wcolor.html Definitely study Johannes Itten's seven colour contrasts.

What exactly do you mean? Do you mean the color theory? Or technical questions? Coloring with watercolor, colored pencil and digital medium have very different approaches. Do you mean decisions about color schemes? Do you mean light and shadow? If you mean basic understandig, there are tons of great videos here (highly recommend Marco Bucci) and on youtube about color theory. What is generally very helpful is to make colorstudies of paintings you like or even fotos. It trains your eye and often you will be very surprised about how greta artists use colors.