Understand that the color of an object will vary depending on the light source. Therefore anyone seeing the object at various times will perceive the color of it differently.

Instead of selective color, try Hue and Saturation, choose Red, and try to tweak the Hue to what you think it should be. Turn out the lights in the room or put a hood over the top and sides of the monitor so nothing distracting is reflected on the screen.

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Is there an instrument out there that you can point at a subject and record the color then use the numbers to get the color to match (fairly) exactly when you edit in, say, photoshop? I take pictures of some roses that are purplish but are much to bright red in the photo. I have used selective color to come close but am not real sure.

Yes, a colorimeter, like the sensors designed to measure the light and color from a display does this, providing a composite and breakdown reading of the light emitting from the display. In addition there are spectrometers that have their own internal light sources that measure the reflective color values of objects. Essentially a spot meter capability in a dSLR camera has a colorimeter capability, but it is not programmed to output standard measurement data, like CieLab scale values.

Great, Thanks. I'll give it a try.

Thanks for the response. I'll look this up and see what they have out there to offer.