Traditionally spot colors are used as single colors, and are not overprinted. However, graphic designers might want to take advantage of the possibility to overprint spot colors. The first time a graphic designer evaluates the color aspect of a design is probably on a monitor. At that stage, chances are that there is no color profiling data available for such overprints. Therefore, the question arises whether it is possible to predict the color of two color overprints without access to printed profiling target information.
Trapping equations can be used as a mathematical model to calculate the overprint color from the spectral densities of the two spot colors, however an estimate of trap and transparency of the second color is required. This research attempted to experimentally find the best average values for trap and transparency that can be used in a trapping equation to obtain a least maximum visual color difference between actual offset and HP Indigo prints and the mathematically predicted color.
Results from 60 different offset printing conditions (6 different inks, different tack sequences, two press speeds, coated and uncoated paper) indicate that, for one mathematical model, a trapping value of 79% and a simulated saturation density constant of 1.4 give predictions that are at most 8.5 DE*94 away from the actually printed colors. 90 percent of the data points are below 5.5 DE*94. These results are good enough to warrant additional investigations, also using prints from digital presses.