Definition: Rewrite each original sentence in order to make it easier to understand by non-native speakers of English. You can do so by replacing complex words with simpler synonyms (i.e. paraphrasing), deleting unimportant information (i.e. compression), and/or splitting a long complex sentence into several simpler ones. The final simplified sentences need to be grammatical, fluent, and retain the main ideas of their original counterparts without altering their meanings.
Input: 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.

Third, some color blind people can only see color if there is enough "mass" of color.  Thin lines seem black, but thick lines can be seen in color.