It seems like these reprints have no clear rules, because I've seen a yellow reprint with an Euroband. Yellow plates still exist on excavators and such. It's weird they chose to do it like this, recently there was a photo of Moskvich with reprinted plates and they look like this, with omitted first letter:
An antique truck still workind as a crane, and it looks as though is was produced just a year ago!
They are pretty rare, all the more reason to cherish! I'm glad Prague still doesn't have any Low/Zero Emission Zones, although it is very likely they'll happen within a few years. Paid entrance to the historic city center (for all non-resident cars) might happen next year.