I always see the days as colored and lined up in a row when I go through them in my head anyway. This is how I see them. I will add the first number that comes to mind, but it may not be 1-7.
Sunday is light pink. 8
Monday is yellow ochre. 1
Tuesday is very light gray-green. 2
Wednesday is yellow-orange. 3
Thursday is light green. 4
Friday is very dark green. 5
Saturday is red. 7
Green is my favorite color, so I guess all the green makes some sense.
These are not rainbow colors but they are what I see in my mind when I'm trying to count days without a calendar in front of me. Strange, I know and not much of a rainbow.