If you have used one of the "quick buttons" (ie, the green check button or the red minus button), the student's writing will be grayed out.
If you sent comments (and did not use either of the quick buttons), then you will see your comments remaining in the comments field.
If a student updates her writing and resubmits it, the student's writing will no longer be grayed out and will return to the normal dark color as before to indicate this student's writing is ready for your review.