濃い and 薄い: strong flavors and deep colors
Compare the intensity of drinks and colors, checking the object rather than always translating 濃い as thick in texture.
The strength of tea and broth
このお茶は味が濃いです。 says that this tea has a strong flavor. 濃い, read こい, concerns intensity and does not necessarily mean a thick texture.
水を足したので、味が薄くなりました。 says that adding water made the flavor weaker. 薄い, read うすい, concerns weak flavor or concentration here, not physical thickness.
Deep and pale colors
濃い青のシャツを選びました。 says that someone chose a deep-blue shirt. The fabric's thickness would need a separate expression such as 厚い.
薄いピンクの紙に書いてください。 asks someone to write on pale-pink paper. 薄い modifies ピンク, so it should not be confused with describing thin paper.
Greater intensity is not always better
私にはこのスープは少し濃いです。 says that the soup is a little too strong for the speaker's taste. 私には identifies whose standard is being applied.
味が薄いですが、まずくはありません。 says that the flavor is mild, but it does not taste bad. Intensity and liking are different judgments.
Must 薄いピンクの紙 be physically thin paper?
Show the answer and why
No. Here 薄い describes the pale pink color. The paper's thickness must be established separately.