The painting mini-game in Episode 6 is when the MC cleans up his room in the DIK mansion, and will happen automatically (assuming you have mini-games turned on).
There is also an optional painting mini-game in Episode 5 at Bella's house. To get it, you need to click on the paint cans near Jill before clicking on Jill to end the free roam.
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If you've had breakfast with Bella and Jill you've missed the chance to paint Bella's fence.
I'm well aware of the walkthroughs, but what does being a "main girl"
mean? We tend to assume it means they are the ones with an endgame, but that's just a guess on our part. Episode 8 was the first time we saw the main girls do something mechanically that someone like Quinn couldn't (though she comes close): the MC can choose them at the crossroad.
My point is that it's entirely possible the crossroad
is the reason the main girls are "main." In which case it could be that by the time we reach Season 4 there will be a new selection of girls that are possible end choices; if so, there's every reason to think girls like Quinn and Zoey could be on that list. I'm not saying it's guaranteed to happen, just that it's a logical possibility we shouldn't overlook simply because Quinn isn't listed as a "main" girl. DPC's walkthroughs frequently omit important information about future choices, after all.
To be fair, the the Episode 5 conversation that is clearly intended to be sexually suggestive. IMHO, trying that hard to be suggestive indicates DPC is trying to mislead us and the eventual explanation will be asexual (at least between Jill and Bella). But that take is just as theoretical as saying the two had an affair. I think the evidence supports it, but we can't prove either at this time.