Need more information than just.. "it was white".
Take some pictures of the sparkplug tips close up and we can give better advice as to what the problem is.
If the insulators were white, and the electrodes were melted.. then you have a problem with preignition, the plugs are the wrong heat range.
If it was a white/gray color, then they're running too hot.
The could be ash on the plugs over-worn, glazing.. all types of things.. we'd have to see it to understand better.