Some examples of student learning exercises related to value.
If someone doesn't have an instinctive sense of how to work with colour the eye can be trained by such exercises, looking at how the value of a hue can be changed, and how different values play together.
A couple of mantras are:
White dilutes. Grey muddies. Black intensifies
If a light, medium or dark value of any hue is used, the same applies.
Another mantra is: value is more important than hue.
I quite often use very dark values of purple, blue, green or brown rather than black to good effect.
One way to develop an eye for colour is to use watercolours and try to make a value scale, or do yarn wrappings. The more you work with colour, the better able you will get at putting effective combinations together.