Edit By Colour by KIRI Engine brings color-based selection into Blender, an approach familiar from photo editing software. You start by picking a color from an object's existing diffuse texture, then refine the selection with threshold values. From there the selection can be smoothed, grown, made smaller, or masked by objects. The add-on is aimed at making selection editing of high-poly or non-quad meshes more manageable.
Once a region is selected, it can be selected, duplicated, split, or deleted directly on the mesh. Color selections can also be turned into face sets for sculpting, or into shader attributes for material masking. A patch-based texturing workflow lets you bake a small portion of the mesh, use that patch to replace selected parts of the model, then re-bake with the original textures into a unified single texture.
For retopology, Edit By Colour can generate automatic quad loops around a color selection, and merging or connecting those loops can speed up manual retopology work. Documentation and a demo video are available.