Are you colouring your hair to cover grey or make it a totally different colour to your own? If yes to either, then you need to stick with a permanent colour - otherwise, you might find that a semi-perm. colour would be better for your hair, condition wise.
If you need to use a perm. colour, are you happy to stick to the same colour for a while? Is yes, then you can get away with just colouring the roots and combing through for the last 5 - 10 mins of developing time. This will help to prevent damage to the ends of your hair - and save some you some money, as you wont need to use a whole treatment each time.
If switching colour, you will obviously need to cover hair entirely.
I find I need to blend in my roots every five weeks, but it really depends on fast your hair grows/colour of hair.
And always use conditioner on your hair when shampooing. Colouring can damage your hair if not treatly it kindly!