My bizhub c451 is still printing only cyan and black after we change toner catridge, what could be the problem please

The problem could be the laser that scans the image or the detectors that see the colors. It could also be a problem with the power supply or the main board. This is not going to be cheap. If you want to continue, you should download the service manual.
http://www.printcopy.info/?mod=docs&brand=Konica-Minolta&gID=15037

The issue might lie with no toner supply to Y and M toner hopper. To check this open front door and pull Y and M imaging units out open the little toner opening mechanism and have a look inside if it has any toner inside.
The motor that turns toner cartridges may have failed.

Option two there is no charge to the drums for MY Imaging unit or IU themselves are over life and failed. (High Voltage board or Power supply is at fault,)

Option three is that the transfer belt primary transfer rollers are broken (effectively Y and M drum is not touching Tranfer Belt)

Option four Laser print head failure.

Option five is to try Trouble reset the machine and do Stabilisation and Background reset in Administrator settings under expert adjustments. Then do Gradation adjust. ( This is usually done when all colour are printing but with bad print quality) {Calibrations}

Http://gethuman.com/phone-number/Konica-Minolta
Konica Minolta Phone Number

Support for Konica Minolta Customers