Any at-home hair-dye experts out there that can offer a little assistance?

I want to temporarily dye my hair for my halloween costume (and also for a little change), and I’ve never used hair color out of a box before. I used to get highlights, but haven’t in years. I want a non-permanent dye that will wash out in 28 or less washes. I bought a box of “Natural Instincts” by Clairol which I think is mostly meant to color grey and washes out in 28 shampoos. My current hair color is sort of a dirty blond/light brown with natural bleaching here and there from the sun. I just looked on the box that I bought (#28 “Nutmeg/Dark Brown”) and I realized that I bought it without noticing that it says “this shade is recommended if your current color is dark brown or black.” So, will this totally jack up my light brown/dirty blond hair? Is there a better brand/color/technique out there for me? Any help wouuld be appreciated. Thanks!