Some things that have worked well in the past (before we could all dress the same):
1 - Philanthropy night. We did an RMH theme with red, yellow, and blue. Everyone was split into three equal groups and told to get a solid color tshirt in red, yellow, or blue. (And we suggested that OLD NAVY or GAP had tshirts that would be perfect! That got around the requirement thing, but still everyone was uniform in cut and style.) One year it was denim bottoms with the shirt, one it was overalls - back when they were cool. Then the room tied in with red, yellow, and blue balloons and nametags in an opposite color.
2 - Song night. Everyone was asked to wear a sundress where the primary color was a shade of blue. Some were light blue, some navy, some royal.... Not all were solid - there were stripes, polka dots, florals, etc. but all dresses were primarily blue and white. Blue is one of those colors where almost all shades work well in a group.
3 - Theme / skit night. We do a country theme. Everyone wears a solid black, fitted tshirt and a dark denim bottom - skirt, capri, jeans, whatever. We do get the shirts all together in bulk to save money and have the uniformity, but you don't have to do that. Then everyone is given a red bandana to add to the outfit. Some make it a headband, some tie it around a ponytail, some use it as a belt.... We are barefoot on that night, but you don't have to do that if you don't want.
4 - One year for an open house round, we all got plain white tshirts and put "big letters" in two colors on them - lettered jerseys I guess they are called.
No specific requirements on color - just two bright colors. Then denim bottoms and a matching ribbon belt.
5 - For preference, if you can get away with it, purchase a ton of the same fabric. We got a beautiful azure blue fabric from JoAnn Fabrics (and got a discount for getting so much). Then the FRC chose a pattern that had three different tops. The bottoms were all the same, but the tops were different. Everyone wore pearl necklaces and cream strappy sandals. We've also done the same thing with a cream fabric.
If I think of others, I'll post! We had a rule that shirts could not be made specifically for a recruitment event, so we just conveniently had really cute lettered jerseys made as a summer house party shirt or some other event. A few people would wear them and be sure to be seen in them, but on the whole, they were used primarily for recruitment! Now we can match shirts with no restrictions, so all the collegians wear the same lettered jersey on Philanthropy night.
