Well as much as I'd like to look good the truth is I had a leak.
Investigation reveals the GPU block o-ring had jumped its groove while assembling. One downside to the acetal backs is you can't see if that's happened.
I think the o-ring will be OK. It was pinched where it exited and re-entered the groove but not badly enough to demand replacement.
Looking at it side on the two pinched bits still stand proud of the groove. So it should still seal. Fingers crossed for test #2 later tonight. lol
Eddy says the second MOSFET block is due next week, so I'll get a spare o-ring sent too.

I don't see any problem with adding the dye later bit by bit. Don't think I'd want to dump a loops worth of concentrate into the res with some brands but a little at a time should get it dissolved evenly through your solution.