From http://www.organognosi.com/skim-and-highlighting-color-codes/

Color codes of this page notes.scpt

--http://www.organognosi.com
tell application "Skim"
set colorMessage to ""
set pageNotes to notes of current page of document 1
repeat with pageNote in pageNotes
set noteColor to color of pageNote
set noteColorText to ""
repeat with codeColor in noteColor
set noteColorText to noteColorText & codeColor & " "
end repeat
set noteText to text of pageNote
set lengthNoteText to length of noteText
if lengthNoteText > 10 then
set noteText to characters 1 thru 10 of noteText as text
else
set noteText to noteText
end if
set colorMessage to colorMessage & "The color code of the note \"" & noteText & "...\" is " & noteColorText & return & return
end repeat
display dialog colorMessage
end tell