more dirty/dirtier

[Vladv1]

"When I took the washing out of the machine it looked dirtier/more dirty than when it went in".
Since we compare one quality in one thing, it should only be "more dirty". Am I correct?

 

[5jj]

Both dirtier and more dirty are correct,

 

[jutfrank]

Since we compare one quality in one thing, it should only be "more dirty". Am I correct?

No. Where did you get this idea from?

 

[Skrej]

The comparative forms of adjectives are primarily based on syllables, with the exception of a handful of irregular ones.

See this article. Note that two-syllable adjectives ending in 'y' can often take either form, although the article doesn't express that.

Personally, I prefer 'dirtier' over 'more dirty', although I wouldn't fret much over the latter.