spatula: yes. It depends, of course, and I'm thinking of past game experience, as well.
When black lines turn grey, they were too thin. They were thin enough at default, and when you shrunk them they were less than a pixel. Seeing that a pixel is as small as you get, they "grey out" instead.
A way around this is to look at the image you want to shrink. Think about what will (and will not) reduce properly, then enhance certain things... If you have, say, a black ring around the image, you can make that black thicker before you shrink it (and it will look better when smaller). Or shrink it then touch it up on the small version, using that as a template.
I'm just thinking off the top of my head here.