Riff Raff Says Paul Wall's Astros Grillz Are 'Hot,' But Ain't His Style

Riff Raff is giving props to fellow H-Town MC Paul Wall for droppin' bank on custom grillz for the Astros -- but he's got 1 minor issue with them ...

THEY'RE TOO BALLER!!

Here's the deal ... Wall promised to make all the World Series champs personal mouth bling in 18-karat gold, which Riff says is dope (and expensive).

But when it comes to grillz, RR told TMZ Sports he rocks with 14 karat 'cause 18s and 24s don't hit the right shade of yellow.

The rapper's more of a shark-teeth-and-diamonds kinda guy ... as you can tell from the glow-in-the-dark joint he had on at In-N-Out.