Hi Will,
Looking at the photo sample you provided, the blue shirt is the only thing that looks a bit out of place, and much of this could be the photo itself. You could maintan the feel of the man's shirt by rendering it as a middle-light grayed down green. or even a white, if you choose a background that works with the resultant value patterns created by the high contrast of vest and shirt. Test this out with the value massing thumbnails. Her clothing is nicely neutral.
I like the use of what appears to me to be natural light, and of course, I don't know how closely this may resemble the resource photo you plan to use. However, if you plan to use an interior -shot resource, yet for the painting, place the figures outside, I think you will run into problems. Likewise, if you don't have full figure reference, or a good stand-in model, I wouldn't suggest inventing the remaining 75% of their bodies.
You need to make a decision about the temperature of your light source. Here, it is cool, and your shadows warm. If you keep the hues in shadow relatively warmer than the hues in light, the light will become more believable for your painting. (This would be the reverse with a warm light source.) Temperature unity also has to follow through in the background and everything else in the picture. The same thing holds true for color saturation: colors in shadows are less saturated than colors in light.
With respect to the color harmony, sometimes it helps to look at your resource in grayscale / black & white. You might consider an analogous color harmony here. I've included some info on Hal Reed's Analogous wheel (terrific tool). Here it just happens to have a dominant orange dialed-in, but of course, you can move it to any dominant hue.
You can order Hal's color wheel through
http://www.artvideostore.com or call Art Video productions at 1-888-513-2187