The colour of the sentences shows Write & Improve’s opinion of the quality of each sentence. Sentences with a white background are sentences that Write and Improve considers good, whereas sentences with a solid coloured background have areas that could be improved. The crossed lighter-coloured background shows that the sentence is better but could maybe still be improved:

Therefore, the colours guide the learner to show where they can most usefully focus their attempts to improve: they should focus on the sentences with coloured backgrounds.

You can see a key describing the feedback given by Write & Improve by clicking on the "Help" button: