In The Color of Mustard, during Johnny's final badminton match, you can hear the announcer say, "Bravo now serving Tungblad". This is a reference to Kirk Tingblad, the director of several Johnny Bravo episodes (including this one). A caricature of Tingblad can also be briefly seen as Johnny's opponent.
Johnny (after he sees he's been transformed into a hamster): Oh no! She made me into a majestic butterfly!
Duck Man: You mean a hamster, right?
Johnny: Right. What did I say?
(After winning the brain-sucking game)Johnny: Ha! Next time you try to suck someone's brain out, make sure he has one! Wait...that didn't out come right.
Even though in previous episodes Johnny is pictured as not being a very good sportive, in the episode The Color of Mustard, it is revealed he is quite good at playing badminton.
Catch that Diggum:
The Diggum are seen in the episode Third Dork from the Sun. They are most likely a spoof on the fictional characters from the Pokemon franchise, Diglett. The title of the round, "Catch that Diggum", is also very similar to the Pokemon catchphrase, "Gotta catch'em all".
The title of the episode Third dork from the Sun is a spoof of the title of the Third rock from the Sun Emmy Award-winning American sitcom, which aired between 1996 and 2001.