diff --git a/dither-image.js b/dither-image.js
index e39b5ea..17efe94 100644
--- a/dither-image.js
+++ b/dither-image.js
@@ -485,6 +485,15 @@ function decimate(input, palette, n, inputError) {
 
     // force to rWb
     //decimated = [0, 0x36, 0x0f, 0x86];
+    
+    let reserved = [0]; // black
+    //let reserved = [0, 15]; // black, white
+    //let reserved = [0, 5, 10, 15]; // grayscale
+
+    let keepers = new Uint8Array(256);
+    for (let i of reserved) {
+        keepers[i & 0xfe] = 1; // drop that 0 luminance bit!
+    }
 
     while (decimated.length > n) {
         let {popularity, fitness, output} = dither(decimated);
@@ -492,7 +501,11 @@ function decimate(input, palette, n, inputError) {
         // Try dropping least used color on each iteration
         let least = Infinity;
         let pick = -1;
-        for (let i = 1; i < decimated.length; i++) {
+        for (let i = 0; i < decimated.length; i++) {
+            let color = decimated[i];
+            if (keepers[color]) {
+                continue;
+            }
 
             //let coolFactor = popularity[i];
 
@@ -513,22 +526,22 @@ function decimate(input, palette, n, inputError) {
                 least = coolFactor;
             }
         }
+
+        let last = decimated.length;
         decimated = decimated.filter((color, i) => {
-            if (palette[i] == 0) {
-                // solid black
+            if (keepers[color]) {
                 return true;
             }
             if (i == pick) {
                 return false;
             }
-            // Also drop any non-black unused colors to save trouble.
-            // However -- this may change dither results.
-            // Try this with/without later.
-            if (popularity[i] == 0) {
-                return false;
-            }
             return true;
         });
+
+        if (decimated.length == last && last > n) {
+            console.log(decimated);
+            throw new Error('this should not happen');
+        }
     }
 
     // Palette fits