Difference between fuel lower and upper fuel injector o ring
 

I am going to change fuel injector o rings. According RealOem RealOEM.com - Online BMW Parts Catalog
upper ring is 13641437487
and lower ring is
13647509752

But what is the difference between those to rings (besides the color) ?

I can order o rings with same specs here but the 2 oem numbers leds to just one ring:
Tätningsring 13641437487 OEM-Koder

Each injector should have two seals one at the intake end and one at the fuel rail. Let me do a little research as it been a while on bmw injectors.

You are right, two seal numbers that can cross over to one part number. The OE ones should have a higher temp rating for the ones in the intake vs the others. Here is one image showing a seal size difference but the kit is after market so one does not know if there is any real difference and the third seal is to reduce the part numbers of the kits so it fits everything from BMW.

Update: For 6 cyl there is a difference in size.

You did not mention application - Year/Engine size
Position: Upper
7.52 x 3.52mm, 1 per fuel injector
2004 BMW X5
Engine designation: M54, Position: Lower, Part: Fuel Injector O-Ring, 9.2 x 2.8mm

In the V8 applications the seals are the same size
06 O-ring 7,52X3,52 QTY 8 PN 13641437487 $3.75
07 O-ring, gray 7,52X3,52 QTY 8 PN 13647509752 $3.98

The grey should be a higher temp version of the same seal.

Next question, Will it interchange, Yes. Will it last as long as you own the car, probably.

V8 Injector image with both seals the same (Aftermarket US)
https://www.rockauto.com/en/moreinfo...411726&jsn=530