How to Fix Color Bleeding in Multi-Color Prints (AMS/MMU)

Color bleeding is the previous filament tinting the next one after a color change: white walls that come out pink because red printed before them, streaks and smears right after a swap, and colors that only clear up several layers into a new section. It happens on every multi-material system — Bambu AMS and AMS lite, Prusa MMU, tool changers with a shared nozzle path — and it is almost always a flushing problem, not a hardware fault.

What color bleed looks like

Three tells separate bleed from other defects. First, the tint always runs in one direction: the color that printed before contaminates the color that prints after. Dark into light is the worst case — red ghosting into white, black graying out yellow — while light into dark is often invisible. Second, the contamination is strongest immediately after a color change and fades with distance: a wall might start muddy at the seam near the swap and clean up a few centimeters later. Third, if the swap happens between layers, you can see a gradient: the first one or two layers of the new color look dirty, then each layer above is a little cleaner as the old pigment works its way out of the melt zone.

If your discoloration does not follow color changes — random brown flecks, burnt specks on single-color prints — that is carbonized debris from the hotend, not bleed. See our clogging guide for that.

Why it happens, in priority order

A single-nozzle multi-material system pushes every color through the same melt zone. When the slicer swaps filament, it purges a set volume of plastic to push the old color out. Bleed means that purge was not enough for that particular pigment pair. The causes, from most to least common:

1. Flushing (purge) volumes too low for the pigment pair. Default purge volumes are a compromise. Heavily pigmented colors — red, black, blue — cling to the nozzle walls and need far more flushing than the default assumes.
2. Flush direction ignored. Purging is asymmetric. Going dark to light needs much more flush than light to dark, because a trace of red in white is obvious while a trace of white in red is not. If you set one global purge number, it is too low for half your transitions.
3. A worn or oversized nozzle. Wear widens the bore and creates dead zones where old melt hides and leaches out slowly. A 0.6 or 0.8 mm nozzle holds more melt and needs proportionally more purge than the 0.4 mm the defaults were tuned for.
4. Wet filament. Moisture makes the melt foamy and sticky, so pigment residue smears through the nozzle instead of flushing cleanly. If the bleed appeared on a spool that used to swap cleanly, dry it — our filament drying guide has times and temperatures.
5. Prime tower too small or disabled. The tower is where the nozzle stabilizes after a purge. Kill it to save filament and the tail end of the old color lands on your part instead.
6. Flushing into infill or objects on the wrong parts. These options recycle purge waste into the print. Used on the wrong object, they push tinted plastic where it shows through walls or transparent sections.

The fix ladder: quick wins first

Flushing into infill and objects: when it is safe

Purge waste hurts, so slicers offer to dump part of it inside the print instead of the tower. Used carefully this cuts waste dramatically; used carelessly it is a bleed generator.

• Flush into objects’ infill is safe when the infill is fully hidden: opaque walls, two or more perimeters, no transparent or translucent filament anywhere on the part. The multicolored mess stays inside where nobody sees it.
• Flush into objects (sacrificial or don’t-care parts) is safe only for objects whose color truly does not matter — a calibration cube printing alongside, a functional bracket. Never route flush into anything white, light, or see-through.
• If a light-colored part shows faint tinting through its walls after you enabled these options, that is your cause: turn them off for that object, or add a perimeter so the contaminated infill cannot show through.

Bambu Studio / OrcaSlicer: Prepare tab → Flushing volumes dialog and per-object right-click menu → Flush options

Accept a little, engineer around the rest

Even a perfectly tuned single-nozzle system carries a trace of the old melt into the new color; the goal is to make the trace invisible, not zero. The practical loop: slice a small two-color test with your real filaments, check the transition on the prime tower and the first walls after each swap, and adjust only the matrix cell for the pair that still shows colour bleed. Two or three iterations of one-cell changes lands you at the minimum purge that gives clean swaps — which is also the setting that wastes the least filament. Log the values per filament brand: pigment load varies between manufacturers, and the red that needed 400 mm³ from one brand may need 700 from another.