Original: Thirdly, for some color blind people, color can only be distinguished if there is a sufficient "mass" of color:  thin lines might appear black while a thicker line of the same color can be perceived as having color.
Simplified: Thirdly, for some color blind people, color can only be distinguished if there is enough color: thin lines might appear black, but a thicker line of the same color can be seen in the correct color.