Textures and Materials turn pink after installing and activating lightweight render pipeline

You need th change the materials shader from Standard to LightweightPipeline

if Edit-> upgrade dont slove,

For HDRP: Try changing autodesk interactive to any other autodesk interactive materials, then check the box after use color map to load them. then switch back to autodesk interactive default material...it will reload textures for that default material and pink will be gone.

For LWRP: Do like above, play with other LWRP materials till your old texture channels loaded in any of LWRP material.

F* hell this one sucks, took me a while and lot of reading BUT: It worked to delete all materials and re-import them from the asset-store. Or in try every shader till finding one that works.

Chancing pipelines only made it worse and was at the root of the problem, after downloading a Unity (company) asses that used some Lightweight Render Pipeline stuff. So next time i will be careful and open all assets in a empty project first!

But stuff still looks like this in the project window (not in the project). Any idea's?

This is because the URP cant recognizes your material, the easy solution proposed by osamansr2o1oo is valid, my only advise is not to upgrade all project material to URP, make sure to upgrade the material as you go thru the project, some materials will not get upgraded hence you might not see that failure. chose the other option

Upgrade selected Material