You can change the transparency of the paper of any layer (independently of changing the opacity of the whole layer).
Open your layer stack, click the left-pointing arrow to bring up the layer menu.
Select 'Edit paper settings'
Using the opacity radial you can change the transparency of the paper, on any layer, to anything you want. You can also change paper colour, metallics, roughness and so on.
If you like you can have wildly different paper textures on each layer - if there are special paper effects you want.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"