I need to allow a group of very pale skinned aliens (albeit with typical mammalian physiology) to become blue reasonably quickly. They are identical in all respects, except the blue skin, to another group of aliens in whom blue skin evolved. The non-blue have to be able to pass as blue when they want to.
Turning blue could be by ingesting something, or by dyeing the skin. The method used should be something that is considered to be relatively non-toxic. I'd like to avoid the idea of a paint.
I vaguely recall reading the the ingestion of methylene-blue causes the skin to look blue but I have been unable to locate any source to verify the idea or to give any indication of its feasibility. Any ideas? Note: if it wouldn't be possible for an ordinary human in the real world to use the method, then it's probably not going to fit my world.