Hello Vande,
That's not that difficult.
All 230/250 and early 280 SL's rims were only primed - in grey. Inside and out.
The outside (road side) was additionally sprayed in body color but the inside (hub side) was not.
Thus, there is no body color visible on the inside,
other than some spay coming through the slotted holes of the rim.
The spare wheel looks exactly the same as the other four ones.
Here is a pic of an untouched car - a 230 (not mine).
Achim