Most small pixels frequently hidden by colour (7)
I believe the answer is:
tiniest
'most small' is the definition.
('tiny' can be similar in meaning to 'small')
'pixels frequently hidden by colour' is the wordplay.
'frequently' indicates alternate letters (letters selected at a regular frequency).
'hidden by' is an insertion indicator (inserted letters are hidden inside).
'colour' becomes 'tint' (tint is a kind of colour).
The alternate letters of 'pixels' are 'ies'.
'ies' inserted within 'tint' is 'TINIEST'.
(Other definitions for tiniest that I've seen before include "I sent it, being the smallest" , "Most small" .)