@Paul: I think that next time I will try a 7.0 on my own hair indeed (or as my stylist advised first try a mix of 6.0 and 7.0) and see how that turns out. My stylist also told me that some hair types absorb the color more and turn darker than hoped for. So maybe if I use a 7.0 the match will be perfect.
@ Henry: the idea is appealling. Only thing is that I'm afraid I will end up with a weird looking patchwork.