푸르다 and 파랗다.
The two words above describe colors: "sky, skyblue, and green".
I think even Koreans are also confused about the way using these two words.
Let me explain them.
There is a word in Korean, 풀.
풀 means grass, especially green sprouts.
So, when you think of 푸르다, think about the color of green sprouts.
That is the origin of the word 푸르다.
That's why you'd better say 파랗다 in case you want to describe the color of a bluesky.
When you want to describe the color of bluesky or sea colors, you can use this word, 파랗다.
When you especially want to describe the color of the deep blue sea, which doesn't seem to be transparent, you'd better say it as 새파랗다.
Here, 새- emphasizes on the degree of something.
Think about the deep blue sea. Its colors are more like dark blue (새파랗다) or dark navy, rather than normal blue sea (파랗다).