Henry, I have a question on your photographies, as they are all looking quiet yellowish.
What kind of light do you use when taking your pictures? (natural?, light bulb? which kind ? do you know its temperature?
Can you take a photography of a white piece of paper with exactly the conditions you use and publish it here, or one of your painting with a white paper as background?
Personally for pictures, know I use my smartphone (Samsung), using the function "Picture", nit portrait, but enlargement x 3. The best is to have the background placed at minimum 1-2 inches behind the flat to attract the focus on the flat, not on the background, and I use a WHITE light bulb, or LED, not yellowish (cold white). The light must be distant from the subject to prevent over-exposure of artificial highlights. Best is also to put the smartphone on a support (table, foot) not just in the hand to prevent movement of vibration when pushing on button or a timer of a few seconds.
Also using the enlargement factor, the subject can occupy 70 to 80% of the field of the picture, not a small surface as you do, where you loose in details, résolution.
Of course multiple light sources is the best as described by Jean Paul,
I have a Nikon, but for routine pictures and WIP, too much time for setting, space, ... actual smartphone are becoming really good !! (although not perfect in colour temperature adjustment, to adjust for correct white