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....