diff --git a/lib/filaments.js b/lib/filaments.js
index ce429e4..07eaaf6 100644
--- a/lib/filaments.js
+++ b/lib/filaments.js
@@ -68,7 +68,9 @@ function assignSpools(filaments, spools) {
   for (const slot of slots) {
     for (const spool of shelf) {
       if (spool.material !== slot.material) continue;
-      if (slot.colour === null) { pairs.push({ slot: slot.index, spool: spool.key, d: 0 }); continue; }
+      // A slot that takes any colour is filled last, from whatever is left:
+      // scoring it 0 would let it claim a spool an exact-match slot needs.
+      if (slot.colour === null) { pairs.push({ slot: slot.index, spool: spool.key, d: Number.MAX_SAFE_INTEGER }); continue; }
       const d = colourDistance(slot.colour, spool.colour);
       if (d !== null && d <= COLOUR_TOLERANCE) pairs.push({ slot: slot.index, spool: spool.key, d });
     }
diff --git a/public/app.js b/public/app.js
index 5e2e352..eab9936 100644
--- a/public/app.js
+++ b/public/app.js
@@ -328,7 +328,9 @@ function printability(m) {
   for (const slot of slots) {
     for (const spool of shelf) {
       if (spool.material !== slot.material) continue;
-      if (slot.colour === null) { pairs.push({ slot: slot.index, spool: spool.key, d: 0 }); continue; }
+      // Filled last, from what's left: scoring it 0 would let an "any colour"
+      // slot take the spool an exact-match slot needs.
+      if (slot.colour === null) { pairs.push({ slot: slot.index, spool: spool.key, d: Number.MAX_SAFE_INTEGER }); continue; }
       const d = colourDistance(slot.colour, spool.colour);
       if (d !== null && d <= COLOUR_TOLERANCE) pairs.push({ slot: slot.index, spool: spool.key, d });
     }
diff --git a/test/filaments.test.js b/test/filaments.test.js
index d2e26b5..4ae4b66 100644
--- a/test/filaments.test.js
+++ b/test/filaments.test.js
@@ -130,3 +130,19 @@ test('a slot with no colour is filled by any spool of that material', () => {
   assert.equal(filaments.isPrintable([{ type: 'PLA', colour: null }, { type: 'PLA', colour: null }],
     [{ id: 1, material: 'PLA', colour_hex: '#123456', colour: 'Anything', remaining_g: 200 }]), false);
 });
+
+test('an "any colour" slot takes what’s left, not what another slot needs', () => {
+  // Reported from the live library: editing slot 2 of a bookmark to "any
+  // colour" stole the one white spool, leaving the slot that actually wanted
+  // #FFFFFF empty and the verdict no better than before.
+  const shelf = [{ id: 1, material: 'PLA', colour_hex: '#FFFFFF', colour: 'Jade White', remaining_g: 1000 }];
+  const slots = filaments.assignSpools([{ type: 'PLA', colour: null }, { type: 'PLA', colour: '#FFFFFF' }], shelf);
+  assert.equal(slots[1].filled, true, 'the slot that asks for white gets the white spool');
+  assert.equal(slots[1].by, 'Jade White');
+  assert.equal(slots[0].filled, false, 'the any-colour slot goes without');
+
+  // With two spools, both are filled and the exact match still gets white.
+  const two = shelf.concat({ id: 2, material: 'PLA', colour_hex: '#123456', colour: 'Anything', remaining_g: 500 });
+  const both = filaments.assignSpools([{ type: 'PLA', colour: null }, { type: 'PLA', colour: '#FFFFFF' }], two);
+  assert.deepEqual(both.map((s) => s.by), ['Anything', 'Jade White']);
+});