In reading the documentation it states that the black message color can no longer me indicated in the junk folder. The brown on grey very slight differential gradation colors makes culling through spam messages looking for good ones a slower process than the dark black in previous versions. Is there any way to change the colors of the characters to make the differential greater?
The colors choices are fixed by Mail. You could try unchecking Color the background according to how spammy it is, and then it will either use a gray background (now) or a white background (potential change in a future version).