When painting subjects that appear to the eye to be white, for example clouds, snow, white buildings or white clothing, some artists paint what they think they see rather than what is actually there. Taking out the white and grey paint is not really the answer.
The key is to experiment and to use a multitude of colours to represent your whites - try out different shades of blues and lilacs for shadows, maybe still interspersing with some greys and use these tones subtly so that they are not just put in as stark contrasts to the highlights. Subtle tinges of different colors will really give your painting much more personality and depth.
I can also recommend an in depth tutorial of how to paint realistically in oils