Our Prepress team sent the Overprint warning with your Online PDF proof if there are elements in your design set to overprint which will affect the way colors appear when printed.
Overprinting refers to the process of printing one colour on top of another - for example, overprinting yellow on top of cyan will create a solid green in the final printing.
It is similar to the "Multiply" effect, but often will not preview in a PDF file. When viewing and exporting your design and press-ready files, ensure that "Overprint Preview" is turned on.
The opposite is "knock out": when two objects of different colors overlap one colour is knocked out - this means they will not print on top of each other.
How to remove Overprint:
Adobe Illustrator: Go to the drop-down menu "Window > Attributes" to open the Attributes window, select the object/s and then make sure that you turn OFF the Overprint Fill / Overprint Stroke boxes.
Adobe InDesign: Go to the drop-down menu "Window > Output > Attributes" to open the Attributes window, select the object/s and then make sure that you turn OFF the "Overprint Fill / Overprint Stroke" boxes.
Also ensure that you have turned off 'Overprinting of Black' in InDesign. Go to the drop-down menu "InDesign > Preferences > Appearance of Black", and then make sure that you turn OFF the "Overprint [Black] Swatch at 100%"