Proper camera settings for a picture?

I'm camera stupid. However, I need to take pictures of test tubes with light colored gas in them. I prop them up against a sheet of ultra white paper. No matter what setting I choose when I bring them up on the computer the background is yellowed like an old photograph. These need to go into a word document so would like it to be as white as possible, plus it makes the gas in the tubes hard to discern since they are shades of brown (some very light). Can someone help me figure out how to set up my camera to take a good picture?

The camera is a Nikon D100 and the lens is an AF NIKKOR 18-35mm (it's the only one I have). I do have a copy of the manual but it is over 200 pages and I'm just lost with this. Playing around with settings isn't getting me anywhere, assuming I'm even adjusting the right things.

Are you in a lab under florescent lights? If so press the WB (WB for White Balance) button or go to WB in the menu and choose florescent lights setting. Try some the other settings as well, like tungsten and all that. Or look up White Balance in the instructions' index.

You can try two things: 1) Go to "White Balance" - and then scroll to e.g. "custom setting #11": and take 3 shots of the white paper only in the same light - or try some of the other white balance options until you get the right one.
2) Set the camera to Manual mode and overexpose your image a bit.

Check your camera/manual for white balance (sometimes labelled wb on your camera) move it so the image is more blue (colder) this should even out the yellow tones, to see the gas make sure you light the tubes from the side and let no light spill onto the background.

Use the same lighting as

This is totally a LIGHTING issue. The exact same thing happens with people who put a white sheet behind someone for a portrait, and the sheet is greyish looking instead of white. I can't teach you all you really need to know about exposure here, but I will try to explain what you need to do to get your background white.

You have to overexpose the white paper as compared to the exposure required for the tubes. Your camera, (and all cameras), expose for 18% middle grey. In other words, your camera is "looking" at the scene, it sees the white paper, and brings exposure down to middle grey. You have to put light on the background that is at least two stops brighter than what is needed for proper exposure on your test tubes. So basically, your background will be "blown out", though in a controlled manner.

Here is how you can go about this. Get everything all set up like you want. Try to get some controlled light on your test tubes so they are well lit. Do not worry at all about the background for now. Work with your shutter speed, or aperture and ISO as needed until you have an excellent exposure of the tubes. When you have this exposure nailed, SET IT MANUALLY INTO THE CAMERA. NO auto functions at all or you will never be able to get the background white. Now you need to get more light on the white background. This light needs to be directed as much as possible to the background only, with no spill or disruption of your test tube lighting. Again, the lights on the background need to be considerably brighter than your lighting on the tubes. The camera is set to expose properly for the tubes. The brighter light on the white background will now render as white. You may have to adjust the output or placement of the lights on the background to get it dialed in just right.

Does all this sound difficult? The basics are actually very simple, but actually doing it with any degree of quality is the hard part. It is why pro commercial photographers who take shots like this all day for a living have vast, intricate lighting setups. What appears to be a simple photograph is often something that requires lots of time, skill, and equipment to achieve.

It'a your white balance that is the problem.

different light sources emit light of different color. Incandescent lightbulbs emit a very WARM light (very yellow), fluorescents emit a very cold light (bluish).

Make sure your white balance is set to the right setting and try to minimize the types of light you have in a scene.