As said, the Vulcan's undersides were (for many years) painted Light a/c Grey which looked almost white on the aircraft. Prior to using grey, the aircraft wore gloss white (with grey/green camouflage tops) and this was seen on all aircraft when they were first camouflaged, the underside grey beginning to appear on aircraft in the early 1970's.
The wrap-round camouflage appeared in the early 1980's and was only applied to a few aircraft. This was simply a continuation of the upper surface camouflage (a mirror image in fact) but the Medium Sea Grey was changed to Dark Sea Grey (combined with the Dark green) overall. Most Vulcan's were withdrawn before they reached repaint stage however.
XH558's story is slightly different. It started-off overall white (like all the other B2's) and then received Medium Sea Grey/Dark Green upper surface camouflage with white undersides. By this stage it had been converted into a MRR machine and it was later given a gloss finish, and the underside was changed to Light Aircraft Grey and it remained in this scheme until it was converted into a tanker and subsequently withdrawn.
After operational use, the aircraft was selected for air display use and it was flown to Kinloss where it was painted in the finish it now has. Although they opted to copy the wrap-round camouflage and colours, they applied a gloss finish (the operational aircraft had a matt finish) so the result looks slightly strange and more contrasty. They also gave the finish a slightly soft sprayed edge in parts and although this was often seen on MRR machines, the overall-camouflaged aircraft all had hard-edged camouflage, so again XH558's finish looks odd in comparison. So that's basically why she is as she is - it was simply a random paint scheme applied at Kinloss for air display flying. The ECM intake was removed when the aircraft was converted into a tanker but this was never replaced so again XH558 is unique in this respect.
It's a shame really that she doesn't look like an operational Vulcan but I guess anything is better than nothing! If I win the Lottery I think I'll be offering to get her painted in proper camouflage with a white underside and full colour insignia - and get that RWR box taken off the tip of the fin!