Why did my colours smudge?

What you see

A small light detail comes out grey or muddy: a yellow nose, a white eye, a flower centre. It happens where the print has just used a dark colour.

Why it happens

When the AMS swaps filament, some of the old colour is still in the hot end. The printer pushes the new filament through (the purge, or flush) until it runs clean, into the chute and the prime tower.

If the purge is too small, the leftover colour tints the first bit of the new one. Going from dark to light needs the most: a little black shows in yellow, but a little yellow never shows in black.

Small details show it first, because they are printed right after the swap and are over before the colour runs clean.

How we found it

On one of our coaster plates, two of four yellow noses came out greenish grey. The purge from black to yellow was only 136 mm³. The file had kept a purge table made for the same colours in a different order, so the numbers belonged to the wrong pairs. Sized from the real colours, black to yellow is 745 mm³, and the noses print clean.

The fix

1. In Bambu Studio, open Project Filaments → Flushing volumes.
2. Read the table: each row is the colour you come FROM, each column the colour you go TO.
3. Check every dark → light pair (black to yellow, white or pink). Around 500 mm³ or more is right; 700–800 for black to white or yellow.
4. Changed the colours in your slots? Press Re-calculate, so the table matches the new colours.
5. For tiny light details, raise the Multiplier to 1.2–1.5.