Flat copy conversion, which is what you are doing is not the easiest thing to get right. I used to do this many years ago but had the advantage of a proper copy set up.
Ideally you should use a simple copy light set up. Now this isn't rocket science but with a single flash unit it may be difficult. Soft natural lighting will probably be a good alternative, but can tend to mute colours.
Things to try.
Photograph you paintings against a dark grey background, that makes them more easy to evaluate.
If you have a polarising filter try that and see if it helps. On the copy set up I used we had polarisers over the light sources as well. You'd be surprised the difference it makes.
You may find that including a reference grey card may help, as it will give you a reference point. Exposure can be critical, and I'd suggest you bracket you exposures and ascertain which gives the initial best result. It will also help in getting a neutral balance. Any good mid grey material will do as a reference,(you don't really need to spend a lot of money in getting a proper one. It's for your reference not anyone else)
I'd tend to use the ACR option within Photoshop as this gives you more control over the image. Use the tone controls to get the highlight and shadows looking as they should. If you are unfamiliar with this drop into Photoshop and use the levels control t optimise the images dynamic range.
If you are in ACR use the Vibrance setting to bring some "vibrancy" into the colours. If in conventional Photoshop add a little saturation.
If you notice some of the colours are not accurate you can use the Hue / Sat controls to adjust them but be careful you can screw up other colours. I'd prefer to use the Hue Sat and Luminance controls in ACR as I think they offer more control.
Hope these comments are of some help
