Wrinkle effect?
You mean this...
If so, its done by putting candy (transparent) colour down, backing with silver, and while the silver is still wet lay some cling film onto it and then take it off straight away....then back it with black. for a finer effect just wrinkle the cling film up and stipple the silver.
Its called 'marbling' in the real world of cars, where they do it to a full size car!! They're braver than me! lol