Can anyone recommend a nail whitener please?

[SunshineCake]

Also, something to stop discolouration after using certain polishes would be useful too. Thank you.

A clear base coat stops colour staining I think?

I did use a base coat and it happened. First time I've had it and it was only a cheap nail varnish on a decent base coat so assumed that it was because of the varnish. I'm trying with two base coats and same brand but lighter colour to see what happens. Thank you.

Perhaps try whitening toothpaste on your nails. Then let us know if it works!