I've dyed my dreads a lot... and its a pain to keep up.
I never go to a salon, always done it myself or had a friend help.
Basically the same rules apply as dying regular hair.. except you can't use conditioner afterwards so regular hair dye tends to be more damaging to dreadlocks.
If you plan to just keep up on your roots it will be a much less tedious process.
If you want vibrant color in your hair you'll have to bleach first... then veggie dye works well for the color, and its actually good for your hair. but it does fade quickly.
You can also use the veggie dye over dark hair for subtle highlights... but again it fades so quickly, to me its hardly worth the mess. (and believe me there is a mess, the color stains everything lol)
Also, Baking Soda/ACV strips a lot of the color out of hair, so you may want to use mostly dr. bronners or dreadlock shampoo bars after you dye.
I've attached a few pics, the first pic my hair was dark blondish, and I bleached a few random dreads. Then I used Manic Panic veggie dye (2 canisters) in Vampire Red and the results are pic 2.
In pic 3 I dyed my hair brown, with a red intensifier... and then after it faded I used manic panics semi-permanent dye in a purple color with more of the red intensifier mixed in. The purple tint faded to brown really quickly though.
If you have any questions I'm happy to help