(Used to work for Clairol) How long do you want it to last? If you really want something TEMPORARY, there are spray-in colors that will wash out in 1 shampoo. Manic Panic and SFX are actually semi-permanent vegetable dyes. Depending on the condition and natural color of the hair, if used on virgin hair (that's what they call it!), they will give you a muted effect and wash out in about a week. On bleached hair, they will give a very bright color and last a lot longer. I put a bright purple manic panic on my natural brown hair, and it was a pretty cherry coke color for a few days. This was a long time ago.

As far as drugstore stuff, the semi-permanent dyes won't do much at all. They are mostly for grey coverage and won't really cause much of an effect on natural color. The demi-permanent actually contain peroxide, and although they do fade, they will probably last a lot longer than you want them to. Both semi and demi permanent colors will not lighten hair, they will only darken and can brighten if you use a bright color.

Hope that helps...