Re: Why do some people's bruises seldom show any discoloration of the skin?

Bruising is due to leakage of blood from small local vessels during injury. 
If these are supeficial, discolouration is usually seen as the blood leaks 
out to just under the skin. If your blood losses are contained by the 
connective tissue sheaths surrounding the muscles or ligaments, it is 
possible for swelling to be slight without much discolouration and yet to 
be  painful of bruising due to stretched nerve endings within the 
surrounding connective tissues.
 It may be that your skin is comparatively less "transparent" than some 
people's or that you, if you'll pardon me, have a thicker layer of adipose 
tissue immediately beneath the skin to restrict the blood leakage at deeper 
levels. Or again, the connective tissue sheaths surrounding your muscles 
and ligaments may be thicker or tougher than other peoples' so that 
although your ligaments and muscles suffer damage, wswelling  and 
local blood leakage at injury, the blood does not find its way out into the 
surrounding tissues. 
Humans are not made from a machine - yet(!)- and have a good deal of 
individual variation in their reactions and proceses.