Working on a swamp piece myself at the moment. I've painted the area vallejo yellow olive 70892, I feel it dry's dark enough, but you could always add a bit of black to the mix. Then I painted the swamp edges with a mix of vallejo lime green 70827 and a dark green and mid brown, this lighter colour on the edges of my swamp I then blended into the yellow olive at its centre. Then I've poured the resin on top, looks good to me, gives a mirky swamp look. I googled lots of images of swamps first, to give me a good Idea of what look I needed to aim for.