"Το πουκάμισο του είναι ανοιχτό."

[Dmatafonov]

why His shirt is light incorrect?

[troll1995]

If you wanted to talk about light color, you would say ανοιχτόχρωμο or ανοιχτό + the color p.e ανοιχτό κόκκινο= light red. Light with the meaning of "not heavy" is ελαφρύ. Ανοιχτό means light only when followed by a color.

[CassieKerr2]

it says 'light' on the translation. How can that not be correct?

[D_..]

Please read troll1995's comment above.