I'm creating isometric buildings in Inkscape. Each building has a base color. To account for lighting, the top of each building has a highlight, the left side has the base color, and the right side has a shadow.
My problem is that whenever I try to pick darker and lighter versions of the base color, one of them always has more contrast with the base color and thus more visual weight. Here's what I mean:
On the left, I create lighting with 20% opacity overlays of black and white. On the right, I simply adjusted the lightness slider on HSL by 20 points (out of 255) in each direction.
The shadow on the yellow example on the left has much more contrast than the highlight. Below it, the highlight on the bluish example has much more contrast. The right side is much better, but I'm wondering what the best way to do things is.
In other words, how can I best make shadows and highlights have equal contrast/ visual weight?