Tricky inclusion match

Okay,
Lets say you have a set of colors and you have a whole bunch of
pictures and you want to match on all pictures who have one or more of
the given colors but you have to at least match on 2 of them (unique,
so not red and red - so a picture could list red twice but that would
not be a match).  I'm trying to use keys and grouping to solve this
but thinking that I might be making this more difficult.  At any rate,
I'm stuck and would appreciate some help : )

Sample Data (expected results below):

<data>
    <colors>
    	<color>red</color>
    	<color>blue</color>
    	<color>fucia</color>
    	<color>violet</color>
    </colors>
    <pictures>
    	<picture sample="1">
        	<color>black</color>
        	<color>grey</color>
        	<color>white</color>
        </picture>
    	<picture sample="2">
        	<color>red</color>
        	<color>green</color>
        	<color>brown</color>
        	<color>blue</color>
        </picture>
    	<picture sample="3">
        	<color>purple</color>
        	<color>orange</color>
        </picture>
    	<picture sample="4">
        	<color>blue</color>
        	<color>green</color>
        	<color>red</color>
        </picture>
    	<picture sample="5">
        	<color>fucia</color>
        	<color>green</color>
        	<color>violet</color>
        </picture>
    	<picture sample="6">
        	<color>red</color>
        	<color>brown</color>
        	<color>red</color>
        </picture>
    </pictures>
</data>

Expected Results (picture matches based on 2 or more colors used and
listed in colors above)

picture sample #2
picture sample #4
picture sample #5