Color Consistency For Product Shots

Question

I'm doing product shots for my wife's eBay sideline.
The problem I'm having is that I will take multiple shots of an item, and despite my best intentions the color will be slightly different in each shot (enough so that when placed side-by-side on a web page, it's clearly and amateurishly different).
Here's some details about what I'm doing:
I'm usually shooting in front of a large window so it's mostly natural light. Sometimes I'll use an overhead flash, but if I do, I do so for all shots. It's not like some shots include flash and some don't. So the lighting should be consistent.
I'm usually shooting against a full white background (or full black)
My white balance is set to Auto
My exposure is manual
I'll usually try to start the shoot with a shot of a gray card. Then in Lightroom, I'll set the white balance for that shot by sampling the gray, and then copy/paste that white balance develop setting to all other shots.
So other than perhaps the Auto WB setting, I'm not sure what I'm doing wrong in the shooting to get different colors.
But that aside, this seems like something that should be fixable in LR, but despite copying the WB setting, I can't figure that out either.
What I would really like to be able to do is use the eyedropper tool on Image 1, then sample what should be the same color (but isn't) on Image 2 and say "LR, do whatever you need to in order to make the sampled color in Image 2 the same color as the sampled color in Image 1".

Answer 1

Gray cards (unless designed specifically to be so) are exposure guides, not color temperature guides.  A white balance target would do you a better job.
The gray card is from Scott Kelby's Lightroom book, and he specifically mentions using it for color correction (actually, the technique for copy/pasting the WB settings gathered from the card is from his book, too).
Also, in the Lightroom manual, it says "Move the White Balance Selector into an area of the photo that should be a neutral light gray. Avoid spectral highlights or areas that are 100% white."
It is the color of the background that varies that gives it away in a grid of images.
When I get home, I'll try to attach some sample images. But in general, the backgrounds are pretty consistently solid white - it's the coloring on the product that's different. Depending on my time and how expensive the item is, I'll use Photoshop to ensure the background is 100% white.
Without a constant light source (window light changes over the course of time) you really can't depend upon copying and pasting the settings.
Yeah, I know. I'm trying to convince my wife to use her eBay profits to buy me a properly lit mini-studio, but until then, I'm stuck with what I have (referring to the situation, not my wife!). But also, the shots are usually within a few seconds of each other - it's not like the light is changing that much, at least not enough to account for the differences.
I have a feeling that it might actually have to do more with exposure. Like the color balance might be OK, but one shot is just underexposed and so the colors appear darker. But you'd think that the LR exposure controls would be able to do that, but so far I just can't get it dialed in.