Some lasers (ex. Alexandrite) specifically seek out the dark pigment in hair. These work well when the skin is light, because there is less risk of damaging the surrounding skin.
Other lasers (ex. Yag) do not have such an attraction for the pigment, and can therefore be used with slightly darker skin tones. These typically may be slightly less effective, and may require more sessions.
Bottom line: Your best bet is to avoid tanning if you are interested in laser hair removal. Of course tanning ages the skin prematurely anyway......but that's a whole other topic.
Hope this helps
Chris Peers MD