While I get that it's wasteful to paint cars before it's necessary, it seems they could try to put together consists with cars with the same liveries – or at least blocks of adjacent cars with the same livery. Honestly, it doesn't really matter.
I do find having different era cars in a consist a bit awkward when travelling from car to car. The other night, I took the 23:36 LSW GO from Exhibition to Danforth and I got on a car with old livery. The car had a manual mechanism to open the door, and the next car (new livery) had an electric/automatic mechanism. The next was manual, and then electric. As an infrequent rider of GO, I found that somewhat awkward, and I could imagine it to be problematic for new riders. Having consistent door mechanisms between cars would be better; grouping cars (by era/type) might make this less of an issue.
Probably not an issue to most riders, though