Cooling coat for dark dogs ?

Good idea or gimmick ?

I’m just wondering as I’ll be taking Opium on my weekend hikes and I’m concerned that she might overheat, being black. We’re on 10km hikes at the moment but I’ll be up to 20kms by the end of the summer, with or without her.
Of course, I’ll be setting off early morning and she’ll stay home if we’re in a heatwave (to save her paws from hot surfaces too) but during the walk, can these coats really help ?
We stop every few kms to offer water but I don’t know if this can be enough.

Opinions ? Ideas ?

PS. I’m considering investing in the ruffwear swamp cooler which seems to have fair reviews

After this morning’s walk, I might have bitten the bullet and ordered cooling coats for the girls ...

I will post a mini review  

Let us know how they get on with them I can't get Katy to wear her coat when it's raining so I'd have no chance of a cooling coat