This one can be a little complicated. If you want a decal projecting over several materials, you'll need to make clever use of schematic's "copy node(s) to" menu, to use the appropriate nodes in several Octane materials.
You can do the decal over a color by using a composite texture. Here is an example where I'm using a decal that actually has an alpha, projected over 2 materials.
In this example, I'm also using texture locators from the shader tree, which can help you visualise it in opengl (this isn't needed though).