What is the difference between Yellow and White mesh?

Generally speaking there is very little difference in the performance of the mesh unless you are using 90T upwards. Yellow mesh absorbs more light and stops light scattering, this helps expose those very fine lines and intricate details. We use Italian mesh yellow and white, they are great for fine halftones with high resolution and has the greatest possible exposure latitude with unsurpassed protection against light-undercutting.

via Wicked Printing Stuff, your home for screen printing equipment and supplies – FAQs.