Start your color application in those hard to color areas like the temples and hairline. This will assure that the product is on those stubborn areas the maximum time. You can also mix a small amount of color, apply it to those areas only, then time for 15 minutes. After that, mix the rest of the product and reapply to those areas first and then to the rest of the hair. This can really help by maximizing the time the color is on the resistant areas.
For better grey coverage make sure of the following - the hair is clean - enough color is on the hair that is stays wet during whole processing time - you can reduce the water by 1/2 ounce to make stronger pigment - you can increase the time on the roots for an hour rather than 45 minutes - try any of these or a combination.