Mehron is a popular brand. They have both water-activated and creme formulas, so you can take your pick. It's usually pretty highly recommended.
I'm partial to Kryolan myself, and again, they have multiple products with multiple formulas. I like their Aquacolor line, which is water-activated paints. A color like 32B would probably be good here, as that's a nice neutral light grey (and what I used to use for Homestuck cosplay).
Ben Nye is another popular brand, though I'm not sure if they make a suitable light grey straight out of the tin like Mehron and Kryolan do.
I know that Tag has become a popular brand recently, though I'm not sure what their selection of greys is like. This is going to be a water-activated paint with a wax-based formula, so it'll apply a bit different from Kryolan Aquacolor and Mehron Paradise, which are glycerin-based formulas. Wolfe also has a wax-based formula, but as far as I know, no light greys like this.
Honestly, among the major theatrical brands, most products are fairly comparable and it comes down to personal preference and what is available. No matter what you do, be sure to seal your paint, but I'm sure you already know that. ;]
Here's some basic body painting information, including how to choose a type of paint and sealing information: