we have a konica minolta bizzhub c250. It has always made excellent color copies and prints, really great quality.
Recently the colors have started coming out slightly off, certain colors on copies might be lighter than the original, sort of faded looking. (such as the deep red/maroon that our company uses for its logos and headers coming out in regular red). Or in other instances the colors coming out darker.
the machine is actually under contract with a company, so we dont own it and they regularly come clean/maintain it. We had the technician come out to look at it and he said everything was working fine, he did all kinds of color tests and didnt find anything. I myself checked color comparisons using a google image of a color wheel and noticed that it didnt have the issue either. but more complex color combinations such as pictures are still coming out slightly off.
It could be a color cartridge issue not compatible with the firmware in the printer. Suppliers are notorious for altering formulations without informing the end users. The technician who serviced the printer may not even be aware of any changes made in the color cartridges.
These types of clandestine changes are common in the chemicals industry, much to the chagrin and frustration of the customers.
Instead of trying to solve the contractor's problems, have them install a replacement machine.
It helps to retain benchmarks of acceptable quality prints for comparison.
are you comparing it to what is on your screen? printers rarely print identical to what you see on a screen and it will also vary between different screens. There are also colour profiles in programs and in the print driver that will have an effect on colour. Otherwise just get the tech to come out again, I service lexmark/epson/oki/canon/hp printers and we get customers calling us out repeatedly all the time. Maybe ask for a different tech, another person may have another idea about what is wrong.
Also try replacing the toner. It may be that the specific toner set you have is slightly different than the ones you use to have. As for deep red being printed as regular red, check out your magenta and yellow toners.