TL;DR: When a multicolor print becomes one color after slicing in Bambu Studio, the cause is almost always one of five things: the preview dropdown sitting on Line Type, "Flush into this object" being ticked, an embedded part that should be a Modifier, the object list order, or a locked object. None of them delete your paint data. This post walks through each fix in the order worth checking, with links to the forum threads where users confirmed what worked.
Why is the model one color after slicing?
A multicolor model shows one color after slicing when Bambu Studio overrides the color assignments at slice time. The paint data is still in the file. The slicer is either hiding the colors in the preview (the Line Type view), purging other filaments into the object body ("Flush into this object"), or resolving overlapping parts in favor of one of them. That is why the model looks right in Prepare and wrong in Preview: nothing was deleted, a setting is overriding it.
The symptom turns up again and again in two long Bambu Lab forum threads: "Multi color print becomes one color after slicing" and "Any idea why slicing removes color". Between them, users reported five separate causes, and for each one somebody came back afterward to say the fix worked on their machine. That confirmation is the reason these five made the list and other theories didn't.
Work through them in this order. The first takes ten seconds.
Fix 1: check the preview dropdown before touching anything
In the sliced Preview, look at the view dropdown in the top-left of the viewport. If it says Line Type, the preview is coloring the toolpaths by feature (outer wall, infill, support), not by filament, so the whole model reads as one color after slicing even though nothing is wrong. Switch the dropdown to Filament and look again.
This false alarm is common enough that it solved the problem for one user in the second thread outright. After another member pointed at the dropdown, Polymer_Designs wrote back: "bud i cant thank you enough this was driving me crazy". Ten seconds, no settings changed.
Fix 2: uncheck "Flush into this object"
This is the most common real cause. Select the object, open its settings, and look for Flush into this object. When it's ticked, Bambu Studio uses the object's body as a purge bucket during filament changes, so whole regions of the sliced model print in whatever color was being flushed at that height. The model turns muddy or lands as one color after slicing, depending on how many swaps the plate has.
Untick it and reslice. In the first thread, Historadical posted this fix and users piled on to confirm it. scorp508: "Thank you! I was losing my mind." In the second thread, Old_Dog_vs_New_Trick reported back: "unclicked flush into object and now slices with ams again".
Flushing into the model is a real waste saver when it's pointed at the right target. If you want the purge hidden inside the print without eating your colors, flush into the infill instead. There's a full walkthrough in flush into infill on a painted 3MF.
Fix 3: change the embedded part to a Modifier
If your model is an assembly of overlapping bodies, say lettering sunk into a plaque or a colored inlay inside a base, Bambu Studio has to decide which body owns the shared volume. It often resolves that fight in favor of the base, and the embedded piece's color disappears from the slice.
Right-click the embedded part in the object list and change its type from Part to Modifier. A modifier stamps its settings, including filament, onto the volume it occupies instead of competing with the base body. brent319, who had exactly this setup in the first thread, replied "OMG I think that worked!" after making the change.
Fix 4: reorder the object list
Related to fix 3, and confirmed separately in both threads: the order of bodies in the object list decides who wins overlapping volume. Background or base bodies should sit above the colored foreground parts in the list. IKWeb traced a vanished paint job to exactly this ("turned out to be the layer order") and Wol confirmed the same reorder fixed it "for me and my X1C as well".
Fix 5: unlock the padlock
Check the object list for a padlock icon on the affected object. A locked object ignores per-object edits, including filament and paint changes, so the model stays one color after slicing no matter what you reassign. Click the padlock to unlock, reassign the colors, and reslice. Lotts hit this one in the first thread and reported the model then "slliced in muliti-color as normal" (typos theirs, relief understandable).
Small painted details have their own failure mode: a feature thinner than your line width can't get its own extrusion, so its color drops out even when everything above is set correctly. Enable Detect thin walls in the print settings, and check your paint job for patches that are physically too small to print before you slice. The gap finder walkthrough covers how to spot them early.
Common questions
Is the paint data gone from the file?
No. In every confirmed case above, the color assignments were still in the 3MF; the slicer was overriding them at slice time. If you want to see for yourself what a 3MF actually contains, drop it on the free 3MF recolor tool. It lists every color in the file and previews them in 3D, in the browser, without uploading anything.
Which of the five is the most common cause?
"Flush into this object" produces the most confirmations across both threads. It's also the sneakiest, because the checkbox can arrive ticked in project files you downloaded from someone else, and nothing in the slice preview tells you purge reuse is the reason your colors vanished.
Does this happen to 3MFs painted outside Bambu Studio?
Yes, the same five checks apply, because they're all slicer-side. A 3MF painted in Layerpaint carries per-triangle color groups in the standard 3MF format; Bambu Studio reads those the same way it reads its own paint, so if the preview shows one color, it's one of the five causes above rather than the file. The first multicolor 3MF guide shows what a correct import looks like.
What if none of the five fixes work?
Update Bambu Studio first. One user in an older thread on the same symptom confirmed a version update cleared it. Then check the object's base filament: unpainted triangles fall back to the base color, which can read as "wrong color" even though the paint sliced fine. If it still misbehaves, post your 3MF in the forum threads linked above; that's where every fix on this page came from.
Try it now
If the model that keeps fighting you has no paint on it yet, that part is the easy bit. Drop the STL on the Layerpaint painter, color it per region in the browser, and export a standard multicolor 3MF that Bambu Studio opens natively. Your first export is free, then $2.97 for your next 3 models, or $39.97 unlimited. No subscription.
Happy printing. 🎨