Overprint a dark color on a light background to give the appearance of a third color. The darker color will print over the lighter ink without knocking it out. There will be a slight change in color of the darker object being overprinted. Allowing two colors to overprint each other will create a "third" or "free" color.

How to Overprint in Indesign
Select one or more Shapes or type with the Selection tool or the Direct Selection tool , or select text characters with the Type tool.

Choose Window > Output > Attributes.
In the Attributes panel, do any of the following:

• To overprint the fill of selected objects, or
to overprint unstroked type, select Overprint Fill.

• To overprint the stroke of selected objects, select Overprint Stroke.

• To overprint a color applied to the spaces in a dashed, dotted, or patterned line, select Overprint Gap.

To view the overprint:
VIEW Menu > Select Over Print Preview

How to create a Transparancy on Indesign (PDF)

Knockouts May Require Trapping
What looks like two overlapping circles prints as a circle and a crescent In most cases, when two objects of different colors overlap they knockout -- they won't print on top of each other.

In the above illustration it looks as if a dark gray circle is printing on top of a yellow circle. However, in reality a gray circle and a yellow crescent shape are printed side by side The yellow knocks out leaving white space for the gray circle to print on.Because what you have are two touching colors there is a chance that the colors can get out of register on the press. Knockouts may require trapping to insure that no unsightly gaps are left behind.

Reverse type is simply white text on a black background. (the reverse of normal black text on a white background).

1. Make sure the text box is selected - click on the fill in the tool bar and bring it to the front.

2. Select black from the Swatches palette to give your text box a black fill.

3. Highlight the text and apply white from the Swatches palette.