In situations like this I prefer to use a camera where I can set white balance to a known "true white" sample, like a sheet of paper. By doing this you greatly improve the chances that the other colors are
true.
I know this probably doesn't provide a quick solution for you.
If you need to take pictures showing true color often, you might want to consider a low-priced but high quality DSLR where white balance can be set to sample, like a Nikon D40.
FWIW.
Best,