The HP Colour LaserJet 1600, 2600 and 2605 series of printers have the same basic internal design and also sadly share a common fault. They are prone to a build-up of toner, dust and dirt on mirrors and lenses inside the printer.
The most vulnerable of these mirrors works closely with the magenta toner cartridge so after these machines have had a fair amount of use this is normally the first colour to be affected. However as laser printers combine black, cyan, magenta and yellow to make colours when you first notice the problem it will probably be because purples are coming out blue or reds and oranges look yellow.
Understandably HP do not supply replacement units due to the construction of the machine but an excellent and detailed guide to analysing and solving the issue can be found online. Despite the fact that this is an incredibly common fault there is very little quality help for it available online so this guide really is a must read for an owner of one of these machines. You can view the guide in full here and be sure to show your appreciation to its creator too.
If you have experience with this problem or have anything else to add please let us know in the comments below.