The global score is not an average of the scores but a weighted mark calculated on the globality of the marks of each criterion. 

The page analysis score is calculated on specific algorithms taking in consideration each criteria score. 

A score between 0 and 40 is red.

A score between 40 and 75 is orange.

A good score is green (75 and more).

« Critical » errors are errors that have to be fixed quickly because they have a direct impact on SEO.