If you want to colour you hair back to its natural level and that is more than 2 shades darker than you currently are you need to do something called pre pigment. Once you know what level your natural colour is you can use the chart above to help select the correct semi permanent colour to create the underlying tone required to support the end result. The reason for underlying tone is to prevent fade and help achieve a more natural, true result.
I would apply the semi to the middle and lengths of the hair missing out the platinum ends as that way you can immediately apply the grey and create the ombre effect. You can apply conditioner to the ends to stop the semi colouring the ends at all.
After processing the semi colour, shampoo and dry your hair before you apply the desired natural level, again leaving out the ends. Apply graphite semi to the ends and process according to the instructions.
You then should have achieved the natural colour with ombre effect, it will probably need a refresh monthly for a couple of months.
Hope this helps