  Adaptive meshing is a fundamental component of adaptive finite element
methods. This includes refining and coarsening meshes locally. In this work, we
are concerned with the red-green-blue refinement strategy in two dimensions and
its counterpart - coarsening. In general, coarsening algorithms are mostly
based on an explicitly given refinement history. In this work, we present a
coarsening algorithm on adaptive red-green-blue meshes in two dimensions
without explicitly knowing the refinement history. To this end, we examine the
local structure of these meshes, find an easy-to-verify criterion to adaptively
coarsen red-green-blue meshes and prove that this criterion generates meshes
with the desired properties. We present a MATLAB implementation built on the
red-green-blue refinement routine of the ameshref-package.
