Why does my color engraving on titanium look different every time I run it?

Yeah, that inconsistency is heat buildup, and the edges behaving differently is exactly what you'd expect. The middle of the scale has material on all sides soaking heat away, but the edges have nowhere to dump it, so they hit the color-changing temp sooner even with identical settings. That's why your golds drift toward blue near the borders, blue usually means the metal got hotter than the gold range wanted.

Slowing down or dropping power won't fix it, it'll just shift the whole problem around and you've alredy found the downside with fine lines blobbing. What helped me on similar pieces was controlling the heat instead of the settings. Break the job into smaller passes with a real cooldown between them, like a couple minutes, and keep the pass count identical every time. Also watch your fill order. If the machine finishes each section completly before moving on, the heat piles up unevenly. A fill that spreads the work around keeps any one area from cooking longer than the rest. And keep the hatch spacing wide enough that adjacent lines aren't reheating each other. On cleaning, fingerprints absolutely cause blotches. Alcohol wipes miss oils, especally if you touched the piece after wiping. I scrub with acetone or a proper degreaser, rinse, then handle it only by the edges or with gloves from that point on. Even a thumbprint laid down mid-session will show up as a weird patch in the color.

Repeatability on titanium is mostly about repeating the thermal story, same clean surface, same pass structure, same cooldowns, same piece temp when you start. If a scale has been sitting near a window or came off a warm workbench, that counts too. I got consistent enough that I stopped treating each piece like a fresh experiment, but it took maybe half a dozen test runs on scrap before the real parts earned thier settings.

One thing that made a real difference for me was bolting the blank down to a chunk of scrap aluminum while I worked, since the extra mass behind the piece pulls heat out and slows the drift enought that the edges stop racing ahead of the middle. Not a magic fix, but it bought me a wider window where the settings stayed in range. I also ran a little test grid on a sacrificial offcut, same design at a few speeds, and just accepted the one that looked best rather than chasing perfect. Titanium seems to have its own oppinion some days.

Before lasers could pull this trick we used to color titanium in an electrolytic bath, a bucket, some washing soda and an old battery charger. Colors came out dead even because the whole piece hits the oxide range at once, no hot spots, no edge drama. Detail work was the pain, we masked lines with tape or a grease pencil. If your pattern is open line work you could laser it lightly, no color, then dip the whole scale so the color follows the marks evenly. Flatter look than heat coloring, but repeatable as sunrise. Ever tried the hybrid route on knife parts, or is the laser-only look what you're after?

Update: clamping the scales to a big piece of scrap aluminum like someone suggested made a huge difference. The edges stopped shifting color before the middle, so the wave lines come out even now. I also gave the blanks a proper degrease instead of just a quick alcohol wipe, and the blotchy spots are gone. Slowed the speed a touch and the fine detail still holds. Thanks for the push in the right direction.