My hair is currently bleached using "Scott Cornwall Decolour Hair Colour Stripper" and bleached using Directions Spring Green but for Comic Con in September and October I'm going as Bucky Barnes and Hawkeye both of whom have short brown hair, which means I can't just wear a wig.
Originally I had planned to just grow out the dye but as the time goes on I don't think I'm willing to face my natural colour after dying it for four years, the brightly coloured hair is my defining feature and an important part of me.
Emotional part out of the way however, I need to temporarily colour it brown but need it to at least mostly come out in one or two washes, I haven't tried sprays or anything since I had brown hair so I don't know how my hair would go with a spray.
Edit: I am willing to bleach and dye it again if a small amount of brown lingers but I would rather not have to do that two months in a row.
a temporary hair colour spray would still work, if you were going black you could have gotten a few cans to test out from poundland but ive never seen any brown. theres some more expensive ones http://www.amazon.co.uk/dp/B00CA2X2JW/ref=pd_lpo_sbs_dp_ss_2?pf_rd_p=569136327&pf_rd_s=lpo-top-stripe&pf_rd_t=201&pf_rd_i=B00129LW50&pf_rd_m=A3P5ROKL5A1OLE&pf_rd_r=1ZKB8XMFN42NTGC9NWTP which should cover your hair easily with a little extra left over for another time. you could also try hair chalks as those show up even on black hair so should have zero problem covering green/other shades
I wonder if a colored gel is a good choice for you since your hair is pretty short. Years ago my husband had bright hair and needed a quick change to a "normal" color for a job interview on extremely short notice. He used gel, and it worked perfectly, and then washed out 100% in the shower. Not sure if there's a brown one, though. Hm... Off to investigate.
I haven't tried it personally but if Roux Fanci-Full Mousse is available to you you might want to try that. I'd probably pick the Lucky Copper to neutrallize the green or else it might look too ashy or greenish. Even if you could only find colored hair gels you could mix up a custom brown from red and green.