I've been reading about Special Effects hair dyes today, and I'm pretty impressed with the quality of these dyes as submitted by the users. These dyes are dominantly temporary hair dyes (as most of the alternative colours giving the vibrant rich shades) yet the colour stays from 6 weeks up to 4 months!
That's quite good, but the best thing about Special Effects is that with every wash the colour doesn't just fade, it kinda gets brighter, revealing vibrant new shade. For example the girl in the photo has used the Special Effects Virgin Rose hair dye, which in the beginning gives quite deep magenta shade, but with every wash the colour gets slightly brighter, and it turns into Neon Pink, so it's still vibrant and rich.
It's also worth noting that with Special Effects shade (as well as Manic Panic, Star Gazer etc. etc.) the final effect depends on the colour base of the hair before dying, if it's pre-coloured, if it's dark or fair, but if you want to get the most vibrant colour it's advisable to bleach it first (with manufacturer's bleaching kit). Also wash the hair with cheap shampoo (which strips the hair clean) and don't use any conditioner prior to dying. Leave the dye to develop according to the instructions on the box, yet with these Veggie dyes (which don't contain ammonia) you can leave it for longer, some users recommended leaving the Special Effects for up to 2 hours for more vibrant shade.
Also good tip: if you have any of the hair dye left over, mix it with the hair conditioner and use it after every wash to preserve the colour for longer!