What is the basis for the colors in the Monokai color scheme for Java files?
Sometimes the type of a method argument is in white, other times it is in light blue
Is there a rationale for that?
The coloring is based on the scopes. You can use ScopeHunter to see the scopes applied to a given word. You will also need to look at the "Java.tmLangauge" files to see the regex that applies a particular scope. There is probably some issue with the regex where the "method" scope is defined.