Answer: 0%
Reason:
As you mentioned on the paper, the percentages listed add to 100%
20+17+17+13+12+11+5+3+2 = 100
So this means there isn't any room for other colors that weren't listed. We cannot go over 100%.
Realistically is there a chance some other color car is out there? Likely. But in this problem's context, it has been ruled out as an impossibility.
If the percentages added to something like 97%, then we'd have a 3% gap for one or more colors that weren't listed.