Always remember when coloring white natural hair, to have 1/3 of your formula in target colors base level in the N/.0 series for a more vibrant effect of target color level & 1/2 of the formula in N/.0 for a more subdued value of target color level. The absence of the R,Y,B pigments in the natural white hair must be replaced or you will get a lighter washed out version of the target color. This is why we need a N/.0 within our formula.
In this case I would go with: 1/3 6N + 2/3 6RR(max) & 20vol developer. After color is applied.... start the 30min processing time....mix up a little 20vol+ bleach & free hand some hi lites over top the color where it will look nice on the outer, top areas of the hair. This will add some slightly lighter accents to the final outcome for depth.