My own map is too dark with hillshading

I am running my own private tile server using CartoCSS as my rendering style. To get hillshading on my maps, I am using a pre-rendered hill shading tile layer from openpistemap.org that I am adding above my own tiles. However, even though I've set a high opacity for that layer, my map is looking too dark and therefore is not really beautiful. I can increase opacity for the hill shading even more, but that will make the shading nearly invisible.

Is there any other way to increase the brightness of a map? Maybe adding a third layer that basically consists of a light color? Any ideas?