I just tested this out. It looks like transparency works with decorations for me... it should as long as the decoration has a transparent background. Can you publish the model or point to a specific part number? Can you recolor the part to anything else?

Hallo B1Trash,

I only choose the part 2437d1 in workshop and the part is brown (not the decoration) colored. I've done it in Firefox and Edge.

OK, here's a screenshot. Recoloring has no effect.

It looks like that is a legacy decorated part, which doesn't have a fully transparent background image. For some reason the decoration image is taking over the part. Scrubs will need to weigh in on this one.

In the mean time, I created a model with that part and recolored it. While it doesn't work in the Mecabricks workshop, I was able to render transparent pieces in Blender using the Cycles render engine. (I did absolutely nothing in Blender to the model except importing it.)

Sorry B1Trash, I'm too fast on posting my problems. The problem exists for the part 21849. I thought the decorated parts are shown transparent in the workshop. OK, now I have a reference and will walk through the shader nodes. Tank You

To be able to use old elements with the second version of the material system there is a real time image processing converter running as a middleware. Transparent parts are the most complicated to handle and if there is a small variation in colour value it doesn’t work anymore and the transparency is gone. It is the same issue with Blender which is a little bit more permissive.

My Problem is solved. The decoration node is using a reference color to create a mask. This color value was set to #eeeeee. I've simply modified the texture with MS-Paint and fill the wanted transparent regions with the reference color. And ha: it works. Thank to all.