Discolouration is often the first visual clue with any clamped off extremity.
As a generality the following clues apply :-
Darkening indicates trapped, uncirculated blood. If you press the dark area you'll see that the point of pressure lightens as you push the blood out and it stays light because no blood is coming back in to feed the capillaries.
Bruising indicates temporary damage to the capillaries near the skin. Bruising is caused by the capillaries breaking and the blood spreading through the area.
Whitening indicates the dying-back of surface capillaries. Recovery from this will depend on the extent of the damage as capillaries can regenerate but cannot get through scar tissue easily.
If an area is looking whiter even when you are not playing then it is a warning for the potential of long term damage and the likelihood of desensitisation as starving an area of blood will also starve the nerve endings of oxygen. In these circumstances refrain from constrictive play until the area is recovered and looking a healthy colour once more.
information verified byDr A.B (GP) and realale999