Since I wrote this up for a friend, I thought I'd post it here. This is not how a hair professional would recommend doing this, and may not work for everyone, but it worked for me. My hair was dark, almost black brown before this process and I'd get vibrant dark red, purple and/or blue when I was done. FYI, I almost always used Special Effects colors, which can be hard to find at times in stores, but stay better than Manic Panic or Raw for this method.
Wash with a clarifying shampoo, no conditioner, and blow dry (this will make your hair the most open to absorbing the color - like untreated dry canvas soaks up paint). The dye will stain your skin and nails - put cream on your face/neck/ears first, and wear gloves to apply. Then apply dye (I use special effects, but pravana is also good if you have a friend at a salon who can get it) and let it dry on your head - I'd wrap it in an old towel and sleep with it in, so 24+ hours. A hint - since this is non-permanent dye, the more thoroughly saturated your hair the better - for my hair it took about 2/3 of a bottle each time; and we'd brush it in, then go back over to make sure everything was soaked.
On darker hair, the darker colors work best - I tended to go devlish (red), deepest purple and/or electric blue. For variegated color do 2/3 to 1/3 and swirl - basically make the color in the bowl look like marble cake batter. But if you have blondish hair or highlights some of the less deep/saturated colors would be great too; just know that different colors fade at different rates, so 'your mileage may vary'.
Once it's dry if you use purple/blue, you'll look like an iridescent beetle is perched on your head - it's kinda beautiful - but regardless of the color it'll be really stiff and helmet like. Rinse thoroughly with the coolest water you can stand - I found a comb helps loosen it up/get it out once you've softened the dye. And it'll take a while to get the water clearish - I don't shoot for clear the first few times; as these are non-permanent dyes, they will bleed a bit. Do not shampoo for at least 24 hours, longer if you can, to give the dye as much time as possible to take.
A hint on rinsing - this dye can stain your skin - so I tend to get in a warm shower and thoroughly rinse my body first so it's less able to absorb it, then drop the temp to rinse out the dye, minimizing the rinse water/body contact. Afterwards, if you wash your body with warm soapy water and a scrubby/loofah most stains will come off quickly (but try to keep soap/scrub away from the freshly dyed areas), and commercial hair dye removal pads are the best for getting your hairline cleaned up without losing the color.
Shampoo as rarely as possible, with color safe shampoo - just rinse and/or "dry" shampoo in between washings. That usually helps the color stay vibrant up to 2 months. You can also add an in-line water filter to your shower head setup (like the Sprite filter we use) - it helps especially if your water is hard or chlorinated.
It also helps to mix some color into your conditioner to keep color vibrant. For "dry" shampoo look in the african american haircare section. Or use cornstarch mixed with crushed matching color chalk to lengthen time between washings but keep hair fresh and non-oily.