It's possible to create a value scale with every color in your palette. Once you've painted a grayscale, it's well worth the time painting a series of value scales with every color you use frequently. Then if you're struggling to get the right tone in a painting, you can easily consult your value scale. (Print this art worksheet for a ready-made grid.)
If you're using watercolor, one way to do this is to gradually add a little more water to the color each time. Or to paint with glazes, creating a series of values by painting a series of blocks, each glazed over once more than the previous block.
With oils or acrylics, the easiest way to lighten a color is to add white. But this is not the only way and not always the ideal as it reduces the intensity of the color. You can also lighten a color by adding another color of a lighter value. For example, to lighten a dark red, you can add a little yellow.
Exactly what colors do when mixed together takes practice and experimentation, but it's time well spent.
