US naval academy (graduation uniforms)

While perusing the USNA’s website, and specifically the graduation photos for the class of 2007, it seems that while the majority of graduates wear the traditional white uniforms, there is a smaller group of grads who are wearing black. What does this mean? Navy vs marines? Virgins vs deflowereds?

Thanks in advance!

Well, in the second picture the collar insignia is pretty clearly that of Marines, while looking at other pics in the site I see the white tunics show no such branch insignia, in the style of the Navy dress white tunic. So probably Navy/Marine.

I will not rely on the Edit window and will add this one separately:

In this pic, you can see that the dark tunic is essentially a stripped-down version of the Marine Dress Blue: Notice the cinch belt and plain epaulets instead of shoulder-boards.

Compare this one, the white is the Navy Dress White tunic: No cinch belt, and shoulder-boards.

Also, on the OPs first (hat-tossing) picture you see the different cut in the jacket vents, and that the male Marines wear a long-sleeve cuffed shirt under theirs, unlike the Navy men.

Thanks - much obliged!