Ju-88g markings question

[digiartist]

I'm looking to create a piece of artwork featuring a Ju88g nightfighter intercepting Lancasters. To make the aircraft a bit bit more visually interesting I want to depict it with spirals painted on the spinners.
From what I've seen in photo's quite a few 88s of Nachtjagdgruppe Norwegen wore them. One in particular I'm interested in is 'B4-FA'. Although the aircraft seems to have it's callsign painted in black it looks to me that the 'F' might possibly be a different colour. Does anyone know if that's likely or is it just a trick of the light?
I've also just recently bought Kagero's excellent Ju88 volume three book. In it are photos of a number of abandoned 88g's wearing tight white spirals on their spinners. All but one of them have had their radar equipment removed and they wear the code prefix of '5U' It is suggested this is a bomber code but these 88's are certainly nightfighters.
Has anyone got any further information about these aircraft? Were they nightfighters that have simply had their radar masts removed, were they used for training or were they attached to a bomber unit?
I'd be grateful for any help in clearing up some of these questions.