How many days of recruitment are there?
If there are four - why not do the colors as a rush-long theme, if you're going to do "Crimson Ball". You could do a tropical theme for Palm Green and a "Heaven/Pearly Gates of ASA" theme for Pearl White, and Gold could be prefs - Going for ASA Gold, maybe? Or an "Oscars" type event, where you present "PNMS with "Gold Statues" (which you obviously take away at the end of the party and return to them on bid day). Or, if you only have 3 rounds of rush, make Pearl White bid day to welcome the new Pearls.
ETA: As for bid day t-shirts, if you do the "Colors" Theme - I'm not exactly sure how to do this without making it look cluttered (maybe y'all have an art major??) but your "slogan" could be "Letter for Letter, We're FOUR times Better!" and then have the 4 colors/exemplars/symbols on the shirts.
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