This one's weird, isn't it. I mean, who goes out with a tin of red emulsion paint and paints everything in town red? Strange.
But it can be explained in idiomatic terms ... thankfully.
It means to celebrate by going into town and having a nice PARTY!!! Woohoo!
(Of course I once celebrated something by getting some tortilla chips and trying to mix salsa, sour cream and grated cheese to make the perfect tortilla dip. I nearly regretted that one.)
Use in Normal SpeechEdit
When warning of a celebration.
"I've passed my driving test, so if I'm late, it's because I'm painting the town red."
When explaining why someone's late.
"Anya passed her driving test today - she's painting the town red. That's why you've missed her, sorry."