There’s more than one way to do this…
Many tutorials advise using the brush in multiply mode (to darken) or in screen mode (to lighten). Or, add layers and set them to multiply or screen mode, depending on the purpose.
Krita offers another way. Suppose you have chosen a medium green color. By pressing the K key, Krita will darken the color…and it will get darker if you press the key again. On the contrary, if you press the L key, the program will lighten the tone.
These methods work for most colors. For some, like red, I like to set the shading / lights tone manually…