You can use “scene color” node to sample pixels behind the material (and from entire screen too) like a texture. You can get ScreenPosition node, break it, invert (1-x) Y axis (green chanell) and then plug it to UV of SceneColor (sample position map). It will make SceneColor sample Y axis in invert
Note that SceneColor require material to be in translucent mode, as sampling behind pixels how translucency work. Also remember that this can only sample pixels that are visible on screen, same as screen space reflection, you might hear about this limitation if you following NVidia’s ray tracing talks. Also remember to use emissive color pin, so what you material outputting is not altered by lightning
If you at it, you can also alter color of sampled pixels so you can play around that too if you want.