I can not add all the images because the account is new and I don't have enough reputation.
I have an object (a tube of a cream) with three diferent materials, the base "plastic" material and I have created two separated materials to add the front and black labels to the tube.
The front and back label materials is the same as the "base material" but I'm trying to overlay the PNG image on top of it.
For doing that I have unwraped the parts of the object where the labels should go and placed the islands on top of the image.
I have created separated UV Maps for that purpose.
Material nodes: For further info in case it's needed here are the screencaps of the material of the front label:
How can I solve this? Thank you!