Painted walls in bathroom with pink paint

Hi. You've not mentioned the condition of this paint and how well it is adhered to the wall or the size and weight of the tiles you intend to place. The safest method due to the inferred age of the paint would undoubtedly be to remove it, either through steaming and scraping or sanding. Alternatively, you could test the paint by applying 12" lengths of ductape and leaving awhile before removing and seeing how much paint this takes off. Obviously if a lot of paint pulls off then it will absolutely need removing. If it survives then you may choose to take the risk of just scoring the paint with a knife prior to priming.

I wouldn't run the risk of a fail. Remove the paint and SBR the revealed surface. Scoring the paint might be ok for a while but there is a risk.