If only it were that simple. If I remember correctly from the Dungeon Master's Guide, it's actually silvery-white with those weird colour pools that function as portals.
My apologies for not managing to find anything,
I briefly considered getting out a silver pencil and just colouring in a piece of blank paper, but our scanner is broken.
Edit: Oh, and there's also haze and clouds.
However, still no pictures.
Edit2: After being inspired by the weather earlier, I was struck by inspiration.
These are the closest I could find:
These are, of course, much cloudier than the Astral Plane is usually.
And nowhere near silver enough.