I'm interested in methods of creating realistic fake bruises (or other injuries, if possible) that will pass a close inspection (possibly the inspection would happen up to several days after the fact if that would help) but either cause little harm, or none at all, to the person who is given them. Bonus points if a person who was given such bruises might be reasonably tricked into thinking they're real.
One one possible option I though of to create fake bruises - or possibly real ones by technical definition, but without underlying injury - by collecting blood (for best results, the person's own blood) and injecting small amounts under the skin. Possibly allowing bruises to be different ages or levels of healing by varying times of injections, as they would change color and "heal" naturally, or making a deeper injury (instead of broader) by repeated injections in the same spot as the bruise "kept bleeding".
The injections marks would need to be accounted for - perhaps using very fine needles and odd placement, or else coming up with some other reason they're there. Also, what about the usual preservatives used when people collect blood - Whatever is standard in test tubes or blood bags (anticoagulant, maybe?) which might show up on a blood screen, even a few days down the road - but would omitting it work, is it necessary to actually collect blood or is it just for longer term storage? Would these "bruises" hurt, and if so, how much? I'd guess they probably wouldn't hurt as much as real ones (I think) but between the needle and the extra fluid and pressure, the area might be sore for a while, maybe.
So my question is, is this idea I already came up with plausible? Are there any obvious reasons why this wouldn't work? Are there methods that might work better, for realistic fake injuries?
Bonus questions - Would a method be detectable at a glance, from detailed visual inspection, or from lab tests - routine tests, while not suspicious of the bruising specifically, or even specific ones testing the bruises - after several days? As the story is set in mildly futuristic world, is there a level of technology that could tell the difference between natural bruises and artificial ones, possibly after several days of healing?
Two possible uses for this trick would be a character trying to appear more injured than they are, to fool someone or spring a trap, or else a character trying to make it appear they've harmed someone else, who they actually don't want to harm. I am specifically interested in if a character woke up with such bruising, would they or someone checking them over assume the bruises were real - especially if the person were also drugged, and maybe a fake concussion (dilating drops in one eye) to explain why they didn't hurt that much, so if there's a better way of doing this, that would also be helpful.