Purple - Missing data, possibly not needed to pass submission, but missing, nonetheless. This may or may not be required for you to fill in. An example of this could be the middle name is missing from the form. It would be shown in purple, but you most likely won't need to fill it in to pass submission.
Yellow - This is usually a metric. The application will notify you of what the usual range is. Yellow doesn't mean you will fail submission; it just alerts you to double-check that specific line item to make sure it is correct.
Red - This is mandatory to fill in, in order to pass submission. Make sure there are no red items when viewing the data completeness check.