My guess is because the old Soviet Foreign entity plates were yellow, and probably kept this colour scheme that people were familiar with? Same as the H at the beginning, also a follow on from Soviet era, just using a modern style base plate.
It was very difficult to find out how, but after I saw someone else posted some I knew it was possible. Eventually I visited the forum and found some text (after translating from Russian, there is no English version) that said about using the Soviet foreign system for upload. It is still complicated because then it will go to Azerbaijan section and does not show up in the gallery for a few minutes so you cannot check your upload until it is ready
Big_Danny_K I don't see them on the site, how did you post them?
All such plates (Soviet, Azerbaijani and Kazakhstani) are uploaded from upload page of the Soviet section. During uploading, they are automatically redirected to particular secton in dependence of regional code (the first three digits). below 501 - Azerbaijan; from 501 to 700 - USSR; above 700 - Kazakhstan.