I spoke directly with the Flameseal manufacturer rep and was told that they won't warranty their product if any stain is applied over their treatment. In my case, I would not be able to get this by a building inspector for the application intended, so have looked to other alternatives, and coming up empty.
I am making my own wiping varnish by following Bob Flexner's recommendation of mixing polyurethane varnish (in my case, Minwax semigloss) with odorless mineral spirits.
Without realising it you actually haven't followed Flexner's advice exactly, see previous Answer, noting carefully the wording of his very first bullet point.
Flexner recommends using ...
The ideal way to do this is to would be to hop into a time machine so you can begin again, and start by doing some colour testing on an offcut or a non-visible part of the board you're working on until you're happy with the result ;-) 1
Since that's not an option here are some alternatives.
Generally the ideal is to colour wood directly, which would ...