I think it is no problem for a girl to wear yukata in grey, especially when obi is red, it sounds very nice. I'm sure you'll look chic in it!
In fact, pink or blue is NOT traditional either - recently there are so many different colours.
Traditional yukata is patterns in dark indigo on the white background (or the other way round) sometimes with a dash of red here and there. But now there are many "modern" patterns are available, with various design.
Young girls wearing in quiet colours such as grey, black, indigo, brown is Edo (not Tokyo) style.