Not all suppliers use the same colour rings, codes and charts.

You are far better to take samples from as many parts of your head as possible and send them to your supplier labelled.
So a sample from as near to your face, one from the side and another from the back.

As Mickeyblueyes has suggested, if you're happy with the colour match of your current system, then supply a sample from that for the top of your head.

Your natural hair may be made up of several colours, not just one colour defined on a colour ring.
Factories then use microscopes to study the hair colour variances and hopefully provide you with the best match & blend.