Did you try desaturating the color on Kirk's shirt to grayscale, then colorizing it? (I assume you're using Photoshop or a similar program.)Putting McCoy's shirt on Kirk worked better than using Spock's, but it turned out to be hard to find pics of McCoy in the standard tunic instead of his lab smock. I tried to change the color of Kirk's shirt, but could never get the color even close. Here's one I thought turned out pretty well, except by the time I broadened it enough to fit Shatner, it made his neck look oddly short.
No, I don't have photoshop. I did these in paint.