I’d say tile adhesive as it gives you more ‘play’ . A sand /cement mix will depend on your mix .
I think it’s one of those things where either method would work tbh . Understand you want to be confident before you commit !
What I meant in times gone by pb a shower enclosure and then simply tiling over it . Slightest of grout failure and the pb soaks up the water and the wall becomes unstable .
You know what happens . Initially you see you can get something for around 200 . Then you look and watch videos / read reviews and see more toys for more £££££.
Ok . So it’s a ‘nearest guess’ for colours ?
What about the finished item - it’s plastic but I don’t want it to fade in direct sunlight nor become brittle .