Color theory covers many aspects of hue, saturation and brightness. The most elementary example of these theories is found in the color wheel. Three basic colors mix to create secondary colors; those six combine to create tertiary colors, and so on. This example works well when creating basic hues with infinite opacity on a blank white background.

The Pantone book is open to the pages of the two colors printed in the images below. Note the middle chip (saturated color) and below will need less white to achieve color quickly.

Unfortunately, this is not the case with screen printing on textiles. Even when printing with pre-mixed inks or those created with an ink-mixing system, the substrate presents a challenge, especially in terms of opacity. Ink manufacturers match color based on a standard printing parameter on a white cotton substrate. This is necessary in order to maintain control of a standard matching requirement. However, it falls short under certain circumstances, namely, when ink is printed on a dark substrate, or worse, a dark polyester blend. Such instances thus require a base plate. This, in theory, seems like a sound solution… until you investigate the possible ramifications of this print parameter.

Deep reflections

In the real world, color we see is reflective light. Light travels into the color and reflects the base and color of the sample we are viewing back to our eye. We see good color when ink is printed directly on a white cotton fabric because the ink absorbs into the cotton and creates a base that is virtually the same color as the top coat. But when color is printed on a white ink base, it is not absorbed, so the white under base reflects through as well. The result is that the overprint color appears to be lighter (less opaque).

To combat this, screen print practitioners adjust screen meshes and printing techniques to deposit more overprint color onto the under-base white. The color will eventually block enough light that it will appear to be the correct color when viewed.

The unfortunate twist to this solution is that different colors require custom print parameters to achieve satisfactory results. Gold, oranges and pastel colors, for instance, don’t require as thick of an ink deposit as blue, purple and red hues. This is generally due to pigment strength and color formulas for each mix—the ink shears once the print starts in production, thus changing the opacity once again.
It’s common for print technicians to adjust parameters during the run to accommodate and, over the years, through trial and error, many are able to struggle through multicolor jobs during setup and print runs. But applying some basic color theory can make the process much easier.

	These two prints demonstrate the under-base theory. Note how the white under-base pushes the color too light, whereas the gray brings the color to the correct tone quickly.

Shades of gray

The color theory lessons we learn early on in life covered basic color and saturation of that color. But the key to success on dark substrates is another wrinkle in color creation—brightness.

Taking color to its basic root, an ink can be created to match a specific hue or saturation. Once a dark substrate is introduced, the brightness of that color is compromised. So if we strip the hue and saturation out of a design, we wind up with a gray study of the art.
We can simulate this in Photoshop by converting the design to Lab mode, then turning off the A and B channels. This leaves all the dark and light information about the colors. Viewing the file in this mode reveals that all color shades require different amounts of brightness to achieve color.

Keeping this picture in mind, close your eyes and imagine all the color in the world, then de-saturate them to the point of no hue. What is left is one true white, one true black and all other colors fall into shades of gray. Here is where a strong argument can be made for utilizing gray as an under-base for most colors, as opposed to a full white.
This is not to say that using a full white under-base will not yield good color results. But, the advantage to utilizing the gray under-base print technique will enable the press technician to hit color quicker, resulting in faster setups and truer color throughout the run.
 

The Image on the left shows the full color design in Photoshop.	This shows the true tints and shades of the color and represents the amount of white needed to hit a color quickly.