My guess is that fields (columns) associated with tasks are black and fields associated with the matters with which the tasks are associated are grey. Noticeably, Status is an exception because it is grey, even though it is a field related to the task.
While the distinction between fields related to tasks and fields related to the matters associated with tasks is surely meaningful to the AppColl developers, it is unimportant to me. I agree with @RichardS3059 that Task reports would be somewhat more readable if almost all text was black.
Date text is a reasonable exception since the difference of blue or red is a useful indicator of whether a task is due in the future or the past.