Is there an optimal way to get all combinations of values in a grouped pandas dataframe?

I think you can do a self merge and query:

df.merge(df, on='ID', suffixes=[1,2]).query('color1 != color2')

Or similar, merge then filter:

(df.merge(df, on='ID', suffixes=[1,2])
   .loc[lambda x: x['color1'] != x['color2']]


   ID color1 color2
1   a    red   blue
2   a    red  green
3   a   blue    red
5   a   blue  green
6   a  green    red
7   a  green   blue
10  b    red   blue
11  b   blue    red
14  c    red  green
15  c  green    red