HP Officejet 6500A prints in B/W but in color it is all yellow

Generally this means one of the ink cartridges or heads is dried out.

Good rule of thumb to go by is never leave ink cartridges of inkjets in heads longer than two years. They can dry out, and require replacing the printer because of the cost of the head being nearly the same as the printer itself.

When all you see is yellow, the magenta and blue are typically problematic, and the black may be too, if you are printing something that is strictly color.

When you travel away from the house, pull out cartridge from the printer to avoid them drying out in the head.

The printer takes 920 cartridge from HP.

Many printers also only support genuine manufacturer ink. Don't use refillable cartridges, or third party ink without carefully reading the reviews.

I used to work at Staples, and learned many people had to find out the hard way. It is always a good idea to buy the warranty with the printer, as a similar printer is usually possible to get as a replacement.

In the printer utility, from Apple menu -> System preferences, or page setup in the file menu of many applications, you can run a utility that does a print head cleaning, which can sometimes declog print heads if you don't think the above warnings apply.

There is usually a date stamped on the cartridge or the box you bought the ink with telling you the age of the cartridge. Make sure it is not past 2 years past expiration. Use a magnifying glass or the camera of your phone to zoom in on it to verify the age.

Lasers are far more expensive up front for color toner, but don't dry out on the head, and the drum is only thing that needs replacing every so often.

UPS, Staples, and FedEx office locations all offer onsite email in printing for people who don't print enough to justify having their own printer.