What do the colors next to lessons mean?

Next to lessons, i see a number of black words, blue, and yellow. what does each one mean?

The top number, with a blue oval, is the number of words in the lesson that the app believes you haven’t encountered before.
The second number, with a yellow oval, is the number of words in the lesson for which “LingQ’s” have been created but which you have not yet marked as known. These would be words you have encountered before in the app and might still be learning.
The third number, with a white oval (is it black for some people?), is the number of words in this lesson that you have already marked as known.
The ratio of unknown words to known words should give you a slight hint as to how difficult the lesson might be for you in comparison to other lessons with a lower or higher ratio.