Salt really has no bearing on it at all, with color or no color.

Colored plaster will have mottling, it will show more where their is crazing cracks ect. The darker the plaster the more of this you can expect. If your ok with that and want darker blue water then go that route.
Gray makes blue water, darker grey makes darker blue till you get to dark then it is just black.

There are many Plaster like products out their that have additives, pozzolans ect to make the product very hard, and last a long time. The one I like is below.

Here is a link that might be useful: Finest finish

Thank you. What is crazing? Visible cracks doesn't sound good. Mottling is fine, but...

A lightish gray is very nice. Don't go too dark, but light gray is definitely nicer than the white.

We just built a salt water pool and went with the gray plaster. We had told our builder we wanted it to look as blue as possible. When he said gray plaster I looked at him like he was crazy and he said gray plaster makes the water look blue. He was right. It looks great. We also installed a color LED light system that is awesome. You can make your pool light up in several colors including multiple hues of blue. It also has preprogrammed light shows which are amazing. Highly recommended.

The blue color of a pool is primarily a reflection of the sky.

Scott