In red, perhaps, some marks appear a shade deeper (6)
I believe the answer is:
darker
'appear a shade deeper' is the definition.
I can't tell whether this definition defines the answer.
'in red perhaps some marks' is the wordplay.
'in' means one lot of letters goes inside another.
'perhaps' indicates anagramming the letters.
'some' says to take the centre.
The centre of 'marks' is 'ark'.
'red' anagrammed gives 'der'.
'der' placed around 'ark' is 'DARKER'.
Can you help me to learn more?
(Other definitions for darker that I've seen before include "Less illuminated" , "More gloomy" , "More deficient in light" , "Dimmer, murkier" .)