The trick to accurate color capture is to always have and use a color reference each time you start shooting in different lighting... but carrying a color reference card big enough to focus on is a pain. As I disclosed back in 2003 (more information at http://aggregate.org/DIT/LENSCAP/ ), the color reference need not be in focus to work -- so you can use a tiny, easy-to-carry, reference. For example, got lenscap?
It should be noted that technically accurate color imaging is very difficult. Photons can have any of a wide range of wavelengths, but most sensors just measure approximately how many photons hit each pixel after passing through the corresponding Red, Green, or Blue filter. Further, human vision employs a mechanism of color consistency which makes the relationship between spectral properties and apparent color quite complex. In short, some very nonlinear things can happen in these transformations, the discussion of which is well beyond this Instructable.
You also should keep in mind that color accuracy doesn't necessarily make good art. People like sunsets, and many other things, in less accurate but more intense "Kodachrome" colors.
Step 1: Making Your Reference
Although cutting-up a color reference card works great, those cards are expensive. Many laser printers produce very neutral blacks which, when printed on a neutral white paper, produce an acceptable reference. For example, just scale the circle shown below to fit within your lenscap, laser print it, trim it, and tape or glue it inside your lenscap.
You can fit at least 4 color reference patches inside a typical lenscap before the area of each color becomes too small to be effective. The colors can be anything, but avoid colors that, as seen by your unaided eyes, appear to change dramatically when you move the reference to different lighting (i.e., try to avoid metamerism). Colors matching your printer's inks are probably not a bad choice. More reference colors allow fancier non-linear corrections of color, whereas a single gray reference really only allows correction of the average color temperature.
No space on the lenscap? Ok. How about a little paint on something? Or perhaps a piece of cloth or duct tape on your camera bag or strap? I used to have a neutral gray pair of sneakers....
Step 2: Using Your Reference
As shown in the figure, simply hold the neutral gray reference in the lighting that is hitting your subjects and sample it to set the "manual white balance." That's it. Jpeg images straight from the camera should be reasonably well balanced until/unless the lighting changes. Of course, don't forget to change the white balance when you use the camera in different lighting (some cameras retain manual white settings across being turned off and on again). Incidentally, you can also use this method to digitally apply any color filter you wish -- simply use a gray card that is tinted the opposite color (e.g., blue tint reference to produce orange filter effect).
The second technique works with any type of camera (even film) and is potentially much more precise, but is more complex and requires post-processing of the images:
If you are using a digital camera, set the white balance to "daylight" (or whichever fixed setting seems closest to correct in the current lighting -- but not "auto") or shoot in "raw" mode. Raw images usually are not affected by the white balance settings in the camera and can yield higher image quality. For a film camera, stick to the same film type, chemical processing, and scanning process.
1. Take a separate photograph of the reference -- possibly out of focus, but filling most of the frame.
2. Shoot as many photos as you like under the same lighting conditions used for the reference.
3. Once you've downloaded the images to your computer, use your favorite image editing program (e.g., photoshop, gimp, cinepaint) to tweak the color balance so that the reference image colors closely match the color values you know they should have. Keep in mind that the RGB values don't have to match, because the reference colors can be slightly under or over exposed. In a color space like LAB or YUV, the L (lightness) or Y (luma, brightness) component might not match.
4. That same set of adjustments is then applied to correct all the photos you shot under the same lighting conditions.
If you made your own color reference, you might not know precisely what the digital values of your reference colors should be for step 3. If so, you can get a usable approximation by taking a photo of them under nice, standard, "daylight" conditions and matching to that. Most cameras and films deliver pretty accurate (or at least pleasing) colors under standard daylight conditions.