You don't have a lot of space on your board for markings. I'm going to assume the following:
- You can't expand the size of the purple board.
- You can't force a pairing indicator on the blue board (e.g. it's made by some other party).
In this case, it's hard to convey precise orientation within these hard constraints. But what you can do with the limited space is signal a warning so the user knows they should pay special attention.
One way to do this is to paint one of the connectors red, so there is a clear indication that (a) there is an attention/warning; and (b) there is a polarity to the socket.
It's common for PCB's to indicate a Pin-1 or Pin-0 number to indicate pin orientation, so the visual cue should make sense to users.
The red indicator isn't going to communicate the entirity of the orientation, but given the extreme limitations on space it should be enough to prompt the user to look at the instructions or otherwise confirm the orientation of the blue board before plugging it in.
If you have the tools, adding an exclamation mark
! or information
i to the red swatch could be additionally communicative.
If you don't want to use red because it may alarm user unnecessarily (e.g. if plugging the boards the wrong way doesn't actually damage them), bright green is a decent alternative.
If you are able to change the markings on the blue board, then it's relatively simple to add clear mating markers to both boards.