Had one yesterday -- black shorts on the one women's team. No issues with undershorts of different colour showing. One girl goes into a slide tackle (good, clean tackle), comes up, and she's got white undershorts on, and the black shorts have been wrenched up the one side and managed to get "tucked into" the top, so that there's white showing.
Ref goes over to her, says something, she fixes the shorts, play on.
Post-game debrief, this gets mentioned. Ref's response "I went over to her, could tell immediately that they were underwear and not undershorts, so I pointed out that they were showing so that she could fix her shorts." Assessor: "How'd you know they were underwear, did she claim that? You should've had her change her undershorts." Ref: "They had little pink hearts all over."
Embarrassed silence for a moment, then onto the next thing!