Lump
There are two airlines flying the 757 in DHL colours, both the old white/red and the new yellow/red. There is DHL Air UK Ltd, based at EMA, operating around 25ish ex BA 75s and EAT (European Air Transport) based in BRU and operating around 10ish 75s as well as a diminishing fleet of 72s and a 15ish strong fleet of A300B4s. Both companies are 100% owned by DHL. Swiftair operates a (diminishing) fleet of 72s, as well as Convairs and Metro's in DHL colours. They also operate Brasilias in their own livery. Swiftair is part-owned by DHL. Then there's SNAS in Bahrain operating Convair's and Metro's in DHL colours and there's DHL Airways in the US operating 72s and A300s, but due to US law DHL holds a 25% stake, which by the way is being sold to Airways management to stop the endless bitching and moaning from the purple and brown brigade. You'll also find airlines in Africa and South America operating aircraft such as Caravans, ATRs and (again) 72s on behalf of DHL. All in all, there are around 230 aircraft worldwide operating for DHL, some like Gemini's MD11 and Islandsflug A300-600F on wetlease.
The reason they're getting yellow is of course the "new" ownership by Deutsche Post. You might also have noticed that DHL trucks and vans are getting a re-spray, and you'd be hard pressed to fail noticing the DHL advertising campaign going on worldwide displaying the new colours. They bought the company fair and square, and to the victor go the spoils so to speak. I wasn't a great fan of the new colours initially, but they sort of grow on you.