Hi Paul, thanks for the answer!
Before I decide on which method to use, I have another question.
The object I'm instancing is a picture frame with a multisuboject material with 2 materials, 1 for the frame (just a white Vraymat) and another for the picture (this is where I'm using the ForestColor map). The problem I'm having is that when the Forestpack is rendered, the whole object get assigned the forestcolor material, so not only the picture gets a random image, but also the frame! What am I doing wrong?