Any light colour is fine, with white giving you the most control over your colour.

Intuitively, adding, say, a brown speedpaint on top of a white primer will maintain the colour brown. If you were to put the brown paint on top of a black primer, it would be brown mixed with black, and combine to be, well, brown-tinted black. This is the case with any paint, since its colourspace is additive, but it’s especially so with translucent paints like speedpaints that show the underlying primer really clearly.

If your only two options are grey or white, I’d honestly suggest white. While I do still think Wraithbone by Citadel is a good middle ground, for a beginner, it is pretty tough to use grey rattle primer since it’s hard to determine coverage and is a fair bit darker than either Wraithbone or white, which can darken your minis considerably.

Here’s a good example from Army Painter as to how primer affects their speed paint colours, as you can see, the most accurate colours are acheived with a white primer. Just be careful, because as mentioned, white primers can speckle like the picture below.

You can’t fully avoid this, but your best way to prevent it is to make sure you have low humidity and no rain, shake the can very aggressively for a very long time, warm it up under warm water if possible, don’t spray in the cold, and spray in short, controlled bursts.