If you're talking about these:
I use to use them exclusively...but never again. In time they begin to dryout and flake...peeling away from the surfaces they were applied. I have several models which I applied it as a means to tint the canopies...green 'sun shield'. I also used the black to 'wash' grills and radiators and it too has flaked and peeled away. Even the surfaces that had 'sealers' applied afterwards had the same effect.
My suggestion is to use the Tamiya transparent colors instead. I haven't had an issue with those.