Only tile grout is to be used. Never plaster. 2 totally different materials with different properties.

Plaster is much more course then grout. With the grout being finer, it fills the joints properly.

Thank you for the feeedback.... Our pool builder said that plaster is used when using transparent tiles so there is no change of color between the grout and plaster. Unfortunately it's done and the pool is filled ... Will this cause issues?

Are you speaking of glass tile? Never heard that remark before that your builder mentioned.

We recommend to our clients a grout that compliments the tile and the plaster. No need to "match" the plaster. On the other hand, white grout and a dark finish on a pool will look terrible.

Time will tell if issues arise. Plaster is meant t be under water 24/7. Seasonal pools however, have no choice.