Every pass leaves a gray streak of metal on it. Even with hard steels like S30V. When it gets loaded up, Dawn power wash cleans it up very easy.
A white plastic pencil eraser will also clean your ceramic. I agree with your approach. Someone who just wants a knife sharp and isn't enjoying the sharpening process for its own sake can get very good results with a good diamond stone, a fine ceramic stone and I would add a strop for those times when you really want the edge to pop.
I bought a diamond stone early on, and am not impressed. Have later ones gotten better? Better than a natural stone?
Good diamond stones are really nice. They are fast and that makes it a lot easier to maintain a consistent angle. If you only need to do 20 passes, it's a lot easier to keep the angle consistent compared to doing 100 passes.
Also, with the natural stone prices going up, diamond stones are starting to look like a better deal than they once might have.
There are some keys to using diamond stones.
1. Buy good ones. The bad ones wear out so fast that even if they are cheap, you can't justify the cost. Outdoors55 on youtube has some good input on buying good diamond stones--and good sharpening stones in general.
2. Don't use a lot of pressure. The diamonds are much harder than the steel and will cut it even without a lot of pressure applied. But they are quite brittle and if you press hard, you will just wear the stone faster. Even doing this, a good diamond stone will last a long time.
3. Don't go too fine on the grit size. The beauty of diamond stones is that they remove metal fast. If you're just looking to finish an edge that's already shaped properly, use your ceramic or maybe even a strop. When you need the diamond stone, you're going to use it to take metal off to reshape the edge significantly--might as well have it do that fast.
4. Follow the manufacturer's recommendations for use and maintenance. In particular, a lot of the nickel-plated steel diamond "stones" specifically advise against the use of water to clean the stone, or against wetting the stone during sharpening because it can cause rusting, causing the nickel that is holding the diamonds in place to flake off.