What is the cause for maple tree leaf veins to be much darker than the leaf itself?

What you are describing is symptomatic of leaf chlorosis. Chlorosis is the general yellowing of a leaf that can be brought on by a variety of factors. Disease, insects, cold weather, pollutants, a high level of minerals, or nutrient deficiencies can cause it. Your best course of action is to conduct a soil test. Call your county MSU extension office and they will help you obtain a kit complete with easy instructions. The results will help confirm or rule out problems in the soil. The test will also check the pH level in your soil. The correct pH level enables your plants to take up needed minerals and nutrients that are available in the soil. Read your test results carefully and if you need any clarification of the information, the extension office will again be glad to assist you.