A screen is essentially printed dots that allow the background to show through. The denser (higher percentage) the screen is, the less background will show through making the ink look darker. For example a 50% screen will let 50% of the background show through so on white paper the ink color will look lighter or different. A 25% screen will allow 75% of the background to show through making the ink color look even lighter still.

When ink is screened its color appears to change. This is a optical illusion due to the fact that more of the background color is able to show through because of the screen, even though the screened dots are still printed in the original 100% ink color shade.

When choosing ink colors for a printing job with screens it is important to consider the above information. If the original artwork has screens you must keep in mind that the ink color will change and possibly pick a darker shade of ink to offset any lightening of the ink due to screening.