A very common way of coating the indoor pool surface is epoxy paint.
It is usually recommended in professional areas due to the low cost
Paint is only applied to pools made of concrete, insulated and pasted with waterproof cement mortar so that their surface is smooth enough to be painted.
The great advantage of coating with paint is that even if a problem arises inside the pool such as peeling or leaking there is no problem since it is repainted on top without removing tiles or mosaics
There are several alternatives to epoxy paints on the market as well as several options as to colors.