The most likely causes are mixed or unstable fluorescent lighting.

First, your copy lights use fluorescent tubes, which can shift color as they age and may even vary along the length of the tube. Replacing the tubes is a good first step.

Second, you may be picking up ambient room light, especially ceiling fixtures, which can mix with the copy lights and create a color gradient. Try turning off other lights in the room and shoot using only the repro lights.

Fluorescent lights can also flicker, so exposure timing may catch different phases of the light output. Using a slower shutter speed, around 1/15 s or slower, can help average out flicker.

To improve evenness, move the lights farther away if possible. Greater distance usually makes illumination more uniform across the subject. If the fixtures stay cool enough, adding diffusion material can also help smooth the light.

So: eliminate ambient light, replace aging tubes, use a slower shutter speed, and increase distance/diffusion for more homogeneous results.