I’ve never tried one colour go but I see your point. Regardless of custom colours, the circle that indicates the last move is the colour of the other side so…
It’s not a perfect solution but you could play with analysis switched on. You could see which was the last move by just pressing ◄. If you don’t wish to use analysis in the game you can simply agree with your opponent not to do so. One Colour Go is already played on an honour basis.
You might also find this interesting: Blindfold 9x9 challenge