The antonyms for the phrase "more well marked" would be less noticeable, faint, unremarkable, and obscure. These words imply that something is not easily identified or distinguishable, lacking in clarity or specificity. A less noticeable or faint feature may be challenging to identify without a closer inspection, whereas an unremarkable or obscure feature may blend in with its surroundings or be completely hidden. It is essential to use precise language when describing something, and employing antonyms can help convey a more accurate and nuanced description of the subject.