Any reason why κόκκινη has two back to back 'κ's?
Basically, because Ancient Greek said so ;)
https://en.wiktionary.org/wiki/κόκκινος (see etymology)
Until now I hadn't noticed back to back consonants in greek (except γγ, which makes the 'ng' sound).
Ελληνικά and θάλασσα come to mind, but you're right, it's not common.
Edit: I thought of a few more...
(But I still stand by my comment that it's not overly common :))
Colors are adjectives and usually agree with the noun in case, gender and number. The word φούστα is feminine (η φούστα) so the color "κόκκινο" becomes feminine. Η κόκκινη φούστα.
Some colors like μπλε (blue) and καφέ (brown) don't change they are what we call invariable. You'll get used to this doing the exercises. And of course you can always ask us.
If you are on a pc you can see all this explained in the Tips & notes. Just look for the image of a key when you begin the lesson.
Ο κόκκινος = masculine singular
Η κόκκινη = feminine sing.
Το κόκκινο = neutral sing.
Οι κόκκινοι = masculine plural
Οι κόκκινες = feminine pl.
Τα κόκκινα = neutral pl
Ο κόκκινος άνδρας = The red man
Η κόκκινη γυναίκα = The red woman
Το κόκκινο παιδί = The red kid
Οι κόκκινοι άνδρες = The red men
Οι κόκκινες γυναίκες = The red women
Τα κόκκινα παιδιά = The red kids