Permanent dye is only one of the methods that you can choose when you are looking to change your hair colour. Chemical dyes can damage your hair and cause it to become frizzy, and the traces of such chemicals are absorbed through the scalp and are detectable in urine. If all you want to do is enhance the colour of your hair or lighten it a little, you have the option to do it gradually with a colour-enhancing shampoo. These shampoos will not completely change your hair colour; they will just enhance it.
Purchase some colour enhancing shampoo and conditioner. There are ones that are especially for lightening and bringing out the natural highlights, and there are ones that enhance the colour you have if you dyed your hair to make it last longer. There are also shampoos that contain henna and naturally deposit colour to your hair over time.
Wash your hair with the shampoo and follow up with a conditioner that is also colour-enhancing and has the same brand and function as the shampoo you are using. The conditioner is just furthering the effect that you want to get.
Use this shampoo and conditioner every time you wash your hair. You might not see the effects immediately, as most kinds of colour enhancing shampoos and conditioners take several uses to get a visible effect. To protect your hair colour, stay away from chlorinated pools and don't use styling products that contain alcohol.