LCD monitors usually have four CFFL tubes , two on top and two on bottom, and they work in pairs.
As they age, they turn from white to darker whites with a bit of yellowish tint.
To replace them, you would have to open the monitor completely, get the whole lcd panel out, turn it around and remove the metal case, then remove the protective glass, the actual lcd panel, and you gain access to the cffl tubes.
After replacement, you also have to be careful to not put too much pressure on the lcd panel or to crack the glass when putting the whole stack back.
See here pictures of how the CFFL tubes look inside, and you can see in other folders on the site various failure modes of cffl tubes :
http://s807.photobucket.com/user/budm/library/Bad%20backlight%20lamp?sort=3&page=1You can separate cffl tubes from stores or eBay but they're expensive and you'd need at least pairs, and ideally you'd replace all 4 at a time.. the boards have current sensors on the two separate outputs ( 2 backlight inverter outputs, two cffl tubes in series per inverter output) and sometimes if a pair uses much more current than the other the backlight is turned off (it's meant to protect the high frequency transformers from overheating when there's too much current used by aging cffl tubes).
So it gets kind of expensive.
You can also buy from eBay LED strips and install two led strips instead of the cffl tubes and there's inverter replacement boards (you just put the board inside the monitor, power it from 12-20v the inverter uses normally and just connect the backlight on/off and adjust light intensity wires from inverter to the new board, basically you're taking the inverter on the power supply completely out of the equation.
With led strips you may get wrong colors as other said above, but it should be cheaper than four cffl tubes.