A nice idea when creating this look is to locate a lot of the alternate colour towards the back, and to leave the very front of the hair matched to her existing colour. This way you will prevent the hair looking stripy from the front, however will still notice the colour when the hair is pulled forward.
When placing the extensions at the back, it is important to ensure they are done in a random manner to avoid the hair looking unnatural or stripy. Using different weights of hair extensions is a great way to do this as suggested, or retipping some of the extensions so that they are a mix of both colours.
When forming your rows, a suggestion for how to prevent them looking stripy would be...
One row of natural colour
One row of a mix of both
One row of just the alternative colour
One mixed row
One row natural with a few alternate colours
One row natural colour
This will prevent the hair looking too symmetrical and unnatural.
Kind Regards