Over the years of being a nail tech, I have used many differernt UV cured topcoats. None of which have discoloured.
When a client has returned to me with discoloured french in the past (I'm talking acrylics here, before Harmony) & I have buffed the surface to produce a lovely white french, it is always apparent to of been an outside influence that has caused the discolouring & not the product.
Certain products we use in everyday life can have this effect like hair mousse for blonde hair (the one that removes yellow tones) can over a few days leave a dirty look to the nail. It took a few weeks to work that out with a client, but that's what it was in the end, as I has the same product at home & tested it.
Also, dye can come of clothing we are wearing, particularly new items, simply by brushing our nails on them throughout the day.
Another clients french white turned pink - it was dicovered she was poorly & had been spending a long time wearing a new red dressing gown, they dye transfered off onto her nail - she went home to check & rang me to say that's how it happened.
This is not a problem to have come up with Top It Off, so I would suggest it's something you are doing, after application.
Another thing to check is your UV lamps bulbs, they do need to be changed every 4-6 months & make sure you are curing for the correct time in the UV lamp - 2mins.
I'm sure we'll work out what's causing this eventually:hug: