Hi there, some time ago I came across discussion of a similar problem. A photographer was trying to accurately capture the color of deep purple velvet and kept getting blue. Mind boggling to be sure!
One thing you can do is to ensure your white balance is correctly set to make sure WB isn't the culprit. Use a photographer's grey card (available at most photo shops) to white balance against in the setting you will be photographing. This will help alleviate white balance issues.
Along other lines, I've read that there's something about royal purple that may confuse the digital camera sensors. Here's a comment from Nikon Support as quoted from another forum.
"... digital camera's have problems with some colors of violet as they are close to the ultraviolet color spectrum. This can cause problems with the CCD of a camera detecting it accurately. This ultraviolet part of the color may not be seen and can result in a blue color rather then purple."
I'll keep looking for more info on this.
