English is actually one of the few languages that describes an adjective before the noun. In most other languages, including Spanish, the noun is stated before the adjective so the direct English translation would be skirt blue
The chart is in the description of the lesson. Most lessons on Duolingo have a description / grammatical explanation.
On the webpage you can easily access it by clicking the lesson.
On the mobile app unfortunately the descriptions do not seem to be available
shirt is camisa, falda is skirt.
Each language has its ways: in English one woudn't say "a skirt blue" because it would be wrong.
In Spanish one says "la falda azul".
There are things one just has to learn.