You could have saved the painting by precolouring the thin set. A bit late for that. A good quality grade latex will do it. Buy the best available. The brand I would recommend is Sherwin Williams.

You will prime the area with a good quality primer like Zinzers.

I wonder why thinset was chosen? It is very porous and may be difficult to not absorb the paint. If it doesn't work I would cover the thinset with unsandable grout mixed very thin and paint it on after you have tinted it or choose one of the pre colored bags.

Try the paint first.