I'm assuming you tried the display> properties> and it wouldn't let you make the change.
Usually this means that your video driver is corrupted and you need an updated diver. If you know the video card manufacturer, you should go to their website and down load the most current driver for it and install it. This usually fixes the problem.
I had a similar problem when I put in a new video card. I used the disk that came with it to provide the drivers and they were already obsolete. Even then, I downloaded newer drivers and still couldn't get it out of 16 colors. I took the video card out and tried to return it and was told that after you have the video card in, most current drivers downloaded, you need to change the adapter refresh rate. For some reason the adapter refresh rate defaults to "unknown" and as long as that's what it is set at you can't get out of 16 colors. I took the card home, reinstalled it and darn if it wasn't true. I changed the adapter refresh from "unknown" to "adapter default". apply>reboot; then changed it from "adapter default" to "optimal" and apply,
reboot. (You have to do this in increments because if you go from "unknown" to "optimal" it defaults back to "unknown". Try right clicking on desktop>properties> settings tab> advanced button> adapter tab and change it if on "unknown". Just worth a try. But keep in mind that you may need updated drivers from maker's web site.
If you have onboard video, you may need the chipset drivers for your motherboard.