We've had this problem on lots of Sanyo projectors and have managed to successfully fix it. Repairing these projectors is possible, but its not an easy job. If anything goes wrong during this process or if you loose or damage any of the parts, it will likely be very difficult if not impossible to get the projector working properly again, so this is perhaps the last thing you should try before replacing it.
The problem is caused - as someone else has suggested - by overheating. Buried deep within the projectors optical path are two lenses that have some kind of colour-correction filters glued onto them. The problem is that the glue that is used to stick these to the lenses, which should be transparent, turns dark after a period of time, probably caused by the heat from the lamp and perhaps also insufficient cooling. This darkening causes loss or reduction of one of the primary colours, resulting in a yellow patch on the image.
The solution is to dismantle the projector and remove the lenses so that the colour correction filters can be removed. The projectors work fine without them - the whites tend to be very slightly off-white, but its very hard to notice.
Be warned that these lenses are buried deep within the projector and dismantling is not a five minute job. If you've not done it before, it'll take several hours, plenty of clear space, a steady hand, a selection of screwdrivers and a sharp craft knife.
After removing the top of the projector, the are two circuit boards layered on top of each other that need to be removed. There are lots of cables that interconnect these boards which will need to be removed, so you'll need to keep a careful note of which cables go where. A digital camera might be useful here.
After unscrewing the top board and unplugging all the cables, it will need to be gently pulled to un-mate the PCB-mount connectors on its underside which push into sockets on the board underneath.
Once this board has been removed, the board underneath can be dis-connected, unscrewed and removed. In approximately the middle of the board, you will find three flexi-print cables that connect to the 3 LCDs on the prism module. Be very very careful with both the felxiprint cables and their sockets. They are very fragile and damage to the cables or sockets will almost certainly render the unit in-operable and unrepairable.
Afte removing the bottom board, you can then very carefully un-screw and lift out the prism module. Whilst you're there its a good opportinity to have a general clean-up, probably with a can of aero-duster or similar to remove all traces of dust.
Once the prism module has been removed, you should be able to identify the two lenses, to the left and the bottom of the prism module, looking from the back of the projector. These are held in place by small metal clips that need to be un-screwed.
After doing this, you will need to loosen all of the screws in the black plastic channeling that surrounds the optical path through the projector. You can then carefully prise the top of the channeling away sufficiently to remove both of the lenses. This isn't very easy, as you need to remove quite a lot of screws and move cabling out of the way, but it is possible if you're careful.
If you've got this far, then you should have the two lenses in your hands. On one of them, you will clearly be able to see the darkening of the colour correction filter.
To remove the filters and the glue that fixes them to the lenses, you will need to soak the lenses in methylated spirit for a few hours and then you can beging to very carefully lift the plastic filter away from the surface of the lens using a sharp craft knife. The best technique is to try to lift the filter at the edges or the corners using the knife, taking great care not to scratch the lens. You should find that the plastic filter itself comes away easily, but the glue that remains will be a lot more difficult to get off and will require extra soaking in the meths and more work with the knife.
By the way, don't worry if the metal bracket glued to the top of the lens comes off. This is used to help hold the lens in place, but isn't really needed - the lenses won't fall out once the projector is re-assembled.
I must emphasise how easy it is to scratch the lenses. You need to be very very gentle with the knife indeed, as even small scratches will quite likely be clearly visible on the image once the projector is re-assembled.
Having cleaned up the lenses, all you have to do it to put the whole thing back together again. As long as you have made a careful note of where all the connectors and screws go, this should be fairly easy. Again, take great care with the flexi-print cables and connectors on the prism assembly.
When you come to re-insert the topmost of the two circuit boards, you need to make sure that it is aligned in extactly the right position, so that the PCB plugs re-mate with the sockets on the board underneath. Also, be very careful that you don't get any of the cabling trapped between the plugs and sockets as you push the two boards back together.
Having re-connected and re-wired you should hopefully now have a fully working projector.