Yes, dark colors heat up more in summer, no matter what they are used for. Dark absorbs heat.

And gravel around a pool? There's probably a very good reason you never see that done: gravel in the pool.

when you say pea gravel do you mean exposed aggregate concrete? Or actual pea gravel? The latter would be a weird choice for lots of reasons.

Darker colors are definitely warmer under foot. How they'll look on the ground will depend on the overall landscape design, but I'd worry that with travertine coping, going too dark will make the contrast between the two jump out at you in a bad way.

I think we need pics of your pool and your choices to have any actual idea.

Yes, I mean exposed aggregate The travertine is English walnut.

If you are talking about epoxy glued pebbles then absolutely not. They are uncomfortable to walk on and even the light ones get hot. When I moved into my house that's what the pool decking was. No matter how hard you try, the pebbles will get loose and end up in your pool. I replaced mine with concrete pavers and we love them. If it's hot enough anything will get hot. But the pavers seem much better than any other pool decking material I've seen. And they are not as slippery.