Haircolors you do at home have two necessary steps before you actually color all your hair—and you need to do them every single time, even if you’ve used the dye before.
The allergy test (because you can develop a sensitivity at any time)
The strand test (make sure the color is what you want).
If you skip either of these, you could end up with an allergic reaction, or with a color you had not expected. Dedicate the time for these tests now to save yourself time and trouble later on.