Well that's also true with 2 colors if the bros just make a checkerboard pattern.

What you mean is that an arbitrary number of bros could section the pool into any arbitrary collection of shapes and with only those 4 ball colors you could guarantee there's a way to fill the sections where each bro-bordered segment has a different color on either side.

Looks like it's bothering him too

Yeah, blue balls are very bothering.

Two for me.

And they probably didn't stop being giddy about it for days.

I will save this image for the very likely situation where someone asks this exact question. Thank you for the suggestion!