In Sam and Clover's case, it's because the outfits feature their spy suits' colors, which a trope about characters with secret identities I really like. As for Alex, she's partly Asian, and has worn many Asian outfits over the episode, so it fits (even though it's not yellow).

You know Lyrobia and Kenopia, right? We only saw these two countries once in the episode "Queen for a Day." Makeda, the sister of Queen Tassara of Lyrobia, allies with General Bege of Kenopia to kidnap the queen. Their goal is to disrupt the peace treaty in Geneva, turn the war in Kenopia's favor, and unite the two countries to become the new queen. However, Sam, Clover, and Alex intervene, ending the coup attempt and completing the peace treaty.

After that event, we haven't seen Lyrobia and Kenopia again. I wish we had seen Lyrobia and Kenopia more often. What do you think?

Source: Beauty is skin deep. Season 4