Light coming through your eyelids, filtered by your skin and blood to give the colour you see. That coupled with images your brain can relate to the random patterns i.e. like seeing the virgin mary on your toast / wall stain etc.

The colour red will be most common when you close your eyes - this is due to the numerous blood vessels and capillaries which your eyelids are coated with.

In an almost complete absence of light, your rods and cones are especially sensitive. This can account for some of the strange shapes you see: the eyes don't switch off, they are actively 'searching' even with your lids closed.

In some cases the imagination may also be held to account.