If we know that half (50%) of the total amount is equal to 6 (since the total is 12) then we find all the combinations of 6 roses.
1. All 6 red roses
2. 3 yellow, 3 white
3. 3 yellow, 3 red
4. 3 white, 3 red
5. 1 red, 3 yellow, 1 white
6. 2 red, 2 yellow, 2 white
7. 2 red, 1 yellow, 3 white
8. 3 red, 1 yellow, 1 white
9. 3 red, 2 yellow, 1 white
10. 3 red, 3 yellow
11. 3 red, 3 white
12. 4 red, 1 yellow, 1 white
13. 4 red, 2 yellow
14. 4 red, 2 white
15. 5 red, 1 white
16. 5 red, 1 yellow
I think that is all, but I hope I helped and good luck!