Gray Plaster - Gray Portland or Dye?

I want a darker finish to my IG. As I'm in Texas I've been told to go with light gray to keep the pool from getting too hot. For a 34x17 pool I've been told the additional cost is nothing to $1K. What is light gray material? Is it just changing the portland in the mix to gray instead of white? That would be less expensive than the white? Is it really adding dye as suggested? Should I expect a mottled look as suggested and would that really be a bad thing? My favorite pools are the dark, stone bottomed looking pools I've seen at the resorts in Hawaii.

I have a grey quartz sand type finish and the only downside is the bleaching out of the steps from the sun in water less than a foot deep. We actually like the motling it looks more natural.