When designing your plastic roll banner or plastic bag art, it is wise to consider trapping as you construct the relationships between / among the colors.
"Trap" is the overlap of two colors when printed side-by-side, also known as a "butt fit."
Colors on your roll banners need to be trapped to each other so that a white space does not appear between them as the colors shift during the press run. Because of the looser tolerances of flexo presses, these overlaps need to be at least 1/8" (9pt).
When printing a dark color and a light color side by side on your plastic banner, always spread the light color into the darker one. This won't result in a visible increase in the size of either color area, because the overlap is usually not seen if the darker color is dark enough.
When printing two light colors side by side, the trap will be seen as a blended "third color" as one color overprints the other. A possible solution to conceal the overlap is to print a black or dark colored outline where the two colors meet. This outline must be at least 1/8" (9pt) wide.
Because of the trap width needed in flexo and its potential (and unintended) visibility, it is recommended that such traps be avoided unless they can work as part of the design of your plastic roll banner.