I have a client who needs a flyer designed and has a printing restriction of one color, in order to keep cost low. To make the design more interesting and appealing I have used the same color blue in both 100% and opacity 60% in Adobe Illustrator. When this project goes to be printed will this qualify for only one color use with the transparency?
Opacity will lighten colors. Using transparency in itself is not generally a problem.
100% color @ 20% opacity = 20% color @ 100%opacity.
It's 6 of one, half dozen of the other.
The difference is interaction.
A 20% tint over 100% (of the same) color means there's 20% color in that area.
20% opacity over 100% (of the same) color means there's 100% color in that area.
Using tints generally knocks out what's under the object to maintain the tint percentage. Using transparency does not create knockouts and color percentages stack to a maximum of 100%.
In addition, blending modes won't have the same effect on a press as they may have on screen when working with a single color. Blending modes can often create complex values for a single color. Those complex values can't always be pulled off with a single color. In a one color piece, although using things like
Multiply will visually alter the objects on screen, when plates are produced and the ink is on press, there's no such thing as "multiply" on the press. So overlapping objects can only ever be 100% solid color at most and never darker. This is often different than what you will see on a digital display.
For one color design it's best to completely avoid altering blending modes and stick to using tints of the single color.
In some cases using overprints can assist. For example, you have a one-color photograph that you want to deepen the overall color by tinting it an additional 20% of the color. You could draw a rectangle over the photo, fill it with a 20% tint of the color and set it to overprint. Then turning on
Overprint Preview in the View Menu will show you what it will look like off a press. Actual artwork makes a difference, but this is similar to using the multiply blend mode at times. However, overprinting is more reliable visually when using a single color because overprinting will never show you a color value darker than 100% of the color.
While using transparency can always lighten an object, it will never darken anything above 100% color regardless of what the screen shows. For my money, it's much easier to use a 20% color @ 100% opacity than it is to use 100% color @ 20% opacity. But ultimately, if you aren't adjusting any blend modes it doesn't really matter.