Dying Dreads With Henna
Dreadlock Styles and Dread Styling
I would recommend to buy at least 500 grams of a henna crop that is very finely sifted, such as Celebration (or any of the crops recomended for African American hair), dye-release them all, and plan to freeze any leftovers you might have. The fine sift will make it easier to rinse off the dreads. A handheld shower head would be fantastic to have, but you could also do a "mermaid soak" (fill up the tub with as much water you need to submerge your head, let the henna paste soften, and softly squeeze/scrub it out of your hair). You might need to change the bath tub water a couple of times. Once most of the paste is rinsed off, stand up under the shower, apply lots of cheap conditioner, squeeze your dreads, and continue rinsing. Shampooing is OK too and will help even further to get rid of the henna "dust".
If you prone to itchy scalp, instead of using citric juices, try dye-releasing your henna with apple juice or cooled green tea. They are much milder.
The application process is the same as often described, only that you want to make sure you squish the mix thoroughly into the dreads. As for timing the application, do a strand test to determine how long you need to leave it on to get the intensity of red you want. I love colored dreadlock like http://hairstylezz.com/best-senegalese-twist-styles/23/ Would you please post pictures after? It'd be great for all of us to see more examples of henna on dreadlocked hair.