red - night vision and hunting (deer do not see red)
uv - make things glow: blood, urin, ...
green and blue - brightest, eyes are most sensitive
white - general purpose, true color
yellow - no special thing, just easy for the eyes, reminds of incan glow
orange - between red and yellow
do not forget that with colroed LEDs you won't see true colors! just shades of the projected LED-color, e.g. green and black and no other colors
bernhard
EDIT:
pink - attracting ladies /ubbthreads/images/graemlins/grin.gif
one more thing: blue light attracts bugs the most, that means white LEDs (using blue dies with phosphor) will also do this. yellow and red is much better in this category.