Here are a few ideas for the 3EB poster, all of these would use similar colors to those in the ad-mat.
This idea is inspired by the tour name 'fall of the summer gods'. A giant greek/roman-inspired bust half-buried in the ground, with a few onlookers to show the scale. I think using the colors form the admat could give this a really nice sense of atmosphere and mystery.
Another direction inspired by the tour name. A few greek columns crumbling, one of them is actually a guitar neck/headstock. I think a more minimal saul bass inspired treatment could look cool for this concept.