I have been digitally photographing artwork of all kinds for several years, and here is what I have learned: Your camera's white balance may be the culprit here, but there are workarounds that may accomplish the same thing.
Whichever program you use to process digital images on your computer usually has a brightness, exposure and contrast control, and by playing with all three of these, you should be able to strike an acceptable balance. Some programs have an automatic enhancement trigger that might work.
When you photograph the drawings, make sure to have even lighting, preferably florescent, because incandescent lights cast a yellow light which is harder to process out of the digital image.