A puzzle in the spirit of the Density™ puzzle. Too simple? Anyway, enjoy!
Final answer: (15)
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I think the answer is
and obtained as follows.
First, look at each individual colour's presence or absence in each of the 5x5 rows to give a 5x5-pixel bitmap. We get, with various orientations:
orange D (or maybe O)
yellow E, M, W, 3
black Z, N
purple S, 5
Now, the bottom row seems to indicate how many of each kind of letter we take; indeed there are 15 coloured pixels there. And I think the only word that uses the given balance of letters is DEFENCELESSNESS. It happens to use the same letter (E) for all the yellow pixels and the same letter (N) for all the black ones.
In comments, jafe very plausibly suggests that perhaps
this word is the longest English word that uses only a single vowel
which seems plausible enough
though often this sort of thing depends on what you're willing to count as a word; e.g., perhaps "defencelessnesses" is even better
and would explain the title.