Can I somehow erase pixels based on how light they are? Perhaps with pressure sensitivity so more pressure erases darker and darker pixels.

You will need to "Edit brush settings," which for me sits in the top left of Krita's top toolbar. On the left side of the window that appears are several categories for editing a brush. Blending Modes should be the second item down under the first category, "General." In the center of the window should be a box with "Favorites" as the first line, which contain what I believe may satisfy your search. Burn or Color Dodge might serve your purpose. I can only recommend to experiment from there.

This link leads to a screenshot of my attempt at emphasis:

https://www.dropbox.com/s/bdb2j5lxcoeu7 ... t.png?dl=0

Thank you. But Blending modes seems to have no effect when using erase.