If you are using the proper hair color which means buying the color tube and developer separate then "YES" you can just color over it.
If you are using a BOXED HAIR COLOR KIT, then I have nothing morre to say......Your hair will be destroyed either way.

Please stay away from those, I get very tired fixing all the mistakes they cause.

Using the proper color and developer will enable you to enjoy the beauty, depth and vitality of gorgeous color....all you have to do is learn a few rules....just like in "COOKING"....and then you can color your own hair with many many wonderful choies.

Try reading the section on HOme Hair Coloring on one of my 2 HAIR Sites.....
Killerstrands.blogspot.com
KillerPics.blogspot.com
You can ask all the questions you want, PLUS request subjects that I will talk about....
Check them out......they have helped many many women