The percentage of white hair will determine the choice of colour
 

For example:

You can achieve 100% white hair coverage with permanent colour below the depth of an 8/0. The colour you select is vital in the result achieved. A semi permanent colour can achieve up to 50% white coverage.

A quasi permanent colour can achieve up to 75% white hair coverage. Semi and temporary colours will blend white hair up to 15%. Please refer to manufacturers instructions for specific guidance.

In some cases an intense depth would need to be used from the colour range e.g. 44/0 or 4/0. Most colour ranges will have specialist colours for white coverage.

Warm/red/copper temporary and semi-permanent colours cannot be used on white hair as the colour result colour will be too bright. 

If hair is resistant you can pre-soften by applying 6% peroxide on the resistant area before colouring, dry it and then colour as normal.

The brighter, warmer shades of some permanent or quasi colours have to mixed with natural/base colours to prevent the final result being too bright.