Color contrast between text and background is very important for perceivability. As colors come closer to each other, they’re more difficult to distinguish. Notice that colors that work well with big font sizes may not for smaller text.

Three blocks of text, each block with three lorem ipsum paragraphs in three different text sizes. The first block is white over black, color contrast of 21 to 1. Works every time. The second block is a ratio of 4.5 to 1 good for the two larger paragraphs but already not enough for the smallest one. The third block has a ratio of 2.9 to 1. It is a bad ratio even for the largest text size.