diff --git a/samples/hp48/index.html b/samples/hp48/index.html
index a4b3bd1..53caab0 100644
--- a/samples/hp48/index.html
+++ b/samples/hp48/index.html
@@ -16,6 +16,20 @@
       --key-text: #f2f2f2;
       --accent: #8cff6d;
       --border: #111;
+      --key-number: #5a5a5a;
+      --key-number-2: #444;
+      --key-arithmetic: #7f5c24;
+      --key-arithmetic-2: #5f4216;
+      --key-trig: #2d6d73;
+      --key-trig-2: #1f4f54;
+      --key-stack: #6b4a8f;
+      --key-stack-2: #4f356b;
+      --key-special: #3d5d8a;
+      --key-special-2: #2c4464;
+      --key-danger: #8a3f3f;
+      --key-danger-2: #632d2d;
+      --key-enter: #5d8a3d;
+      --key-enter-2: #45662e;
     }
 
     * { box-sizing: border-box; }
@@ -124,34 +138,6 @@
       padding: 14px;
     }
 
-    .title {
-      color: #fff;
-      margin: 0 0 10px;
-      font-size: 14px;
-      text-transform: uppercase;
-      letter-spacing: 0.08em;
-    }
-
-    .keypad-labels {
-      display: grid;
-      grid-template-columns: 4fr 3fr 2fr;
-      gap: 12px;
-      margin-bottom: 10px;
-    }
-
-    .keypad-label {
-      color: #d8d8d8;
-      font-size: 11px;
-      font-weight: bold;
-      letter-spacing: 0.08em;
-      text-transform: uppercase;
-      text-align: center;
-      padding: 6px 8px;
-      border-radius: 999px;
-      background: rgba(255, 255, 255, 0.05);
-      border: 1px solid rgba(255, 255, 255, 0.08);
-    }
-
     .keypad-wrap {
       display: grid;
       grid-template-columns: minmax(0, 4fr) minmax(0, 3fr) minmax(0, 2fr);
@@ -185,11 +171,41 @@
       min-height: 46px;
     }
 
-    .keypad button.active-toggle {
+    .keypad button.active-toggle,
+    .active-toggle {
       background: linear-gradient(180deg, #6a8c52, #49693a);
       color: #fff;
     }
 
+    .key-number {
+      background: linear-gradient(180deg, var(--key-number), var(--key-number-2));
+    }
+
+    .key-arithmetic {
+      background: linear-gradient(180deg, var(--key-arithmetic), var(--key-arithmetic-2));
+    }
+
+    .key-trigonometry {
+      background: linear-gradient(180deg, var(--key-trig), var(--key-trig-2));
+    }
+
+    .key-stack {
+      background: linear-gradient(180deg, var(--key-stack), var(--key-stack-2));
+    }
+
+    .key-special {
+      background: linear-gradient(180deg, var(--key-special), var(--key-special-2));
+    }
+
+    .key-danger {
+      background: linear-gradient(180deg, var(--key-danger), var(--key-danger-2));
+    }
+
+    .key-enter {
+      background: linear-gradient(180deg, var(--key-enter), var(--key-enter-2));
+      font-weight: bold;
+    }
+
     .keypad-group button.enter-tall {
       grid-row: span 2;
     }
@@ -230,12 +246,9 @@
       min-height: 40px;
     }
 
-    .error {
-      margin-top: 10px;
-      min-height: 20px;
-      color: #ff8a8a;
-      font-family: "Courier New", monospace;
-      font-size: 13px;
+    .display-error {
+      color: #8a2a1c;
+      font-weight: bold;
     }
 
 
@@ -244,7 +257,6 @@
         padding: 16px;
       }
 
-      .keypad-labels,
       .keypad-wrap {
         gap: 8px;
       }
@@ -265,7 +277,6 @@
         border-radius: 16px;
       }
 
-      .keypad-labels,
       .keypad-wrap {
         grid-template-columns: 1fr;
       }
@@ -297,12 +308,6 @@
       <input id="input" class="hidden-input" type="text" autocomplete="off" aria-hidden="true" tabindex="-1">
 
       <div class="panel" id="keypadPanel">
-        <div class="title">Calculator Keys</div>
-        <div class="keypad-labels" aria-hidden="true">
-          <div class="keypad-label">Functions</div>
-          <div class="keypad-label">Numbers</div>
-          <div class="keypad-label">Operators</div>
-        </div>
         <div class="keypad-wrap">
           <div class="keypad-group functions" id="functionsKeypad"></div>
           <div class="keypad-group numbers" id="numbersKeypad"></div>
@@ -319,7 +324,6 @@
         </div>
       </div>
 
-      <div id="error" class="error"></div>
     </div>
   </div>
 
@@ -331,7 +335,6 @@
     const stackEl = document.getElementById('stack');
 
     const displayEl = document.getElementById('display');
-    const errorEl = document.getElementById('error');
     const modeLabel = document.getElementById('modeLabel');
     const functionsKeypadEl = document.getElementById('functionsKeypad');
     const numbersKeypadEl = document.getElementById('numbersKeypad');
@@ -344,6 +347,7 @@
     let openMenuName = null;
     let isMovingStackItem = false;
     let stackSnapshotBeforeMove = null;
+    let currentErrorMessage = '';
 
     function getKeypadLayout() {
       return {
@@ -427,6 +431,42 @@
       };
     }
 
+    function getKeyClassName(key) {
+      if (key.type === 'input') {
+        return 'key-number';
+      }
+
+      if (key.value === 'clear') {
+        return 'key-danger';
+      }
+
+      if (key.value === 'enter') {
+        return 'key-enter';
+      }
+
+      if (key.type === 'special') {
+        return 'key-special';
+      }
+
+      const stackCommands = new Set(['neg']);
+      const arithmeticCommands = new Set(['add', 'sub', 'mul', 'div', 'mod', 'pow', 'sqr', 'sqrt', 'recip', 'log', 'ln']);
+      const trigonometryCommands = new Set(['sin', 'cos', 'tan', 'asin', 'acos', 'atan']);
+
+      if (stackCommands.has(key.value)) {
+        return 'key-stack';
+      }
+
+      if (arithmeticCommands.has(key.value)) {
+        return 'key-arithmetic';
+      }
+
+      if (trigonometryCommands.has(key.value)) {
+        return 'key-trigonometry';
+      }
+
+      return '';
+    }
+
     function renderKeypadGroup(groupEl, layout) {
       groupEl.innerHTML = '';
       const occupied = new Set();
@@ -449,6 +489,10 @@
 
           const button = document.createElement('button');
           button.textContent = key.label;
+          const keyClassName = getKeyClassName(key);
+          if (keyClassName) {
+            button.classList.add(keyClassName);
+          }
           button.style.gridColumn = String(colIndex + 1);
           button.style.gridRow = `${rowIndex + 1} / span ${key.rowspan || 1}`;
           if (key.rowspan && key.rowspan > 1) {
@@ -591,7 +635,8 @@
           execute(key.value);
         }
       } catch (error) {
-        errorEl.textContent = error.message;
+        currentErrorMessage = error.message;
+        render();
       }
     }
 
@@ -719,17 +764,23 @@
       }
 
       stackEl.innerHTML = lines.join('');
-      if (calc.isEditing) {
+      if (currentErrorMessage) {
+        displayEl.textContent = `ERROR: ${currentErrorMessage}`;
+        displayEl.classList.add('display-error');
+      } else if (calc.isEditing) {
         displayEl.textContent = `ENTERING: ${calc.inputValue}`;
+        displayEl.classList.remove('display-error');
       } else if (isMovingStackItem && hasStackSelection()) {
         displayEl.textContent = `MOVING: ${['X', 'Y', 'Z', 'T'][stackCursor] || '?'}`;
+        displayEl.classList.remove('display-error');
       } else if (hasStackSelection()) {
         displayEl.textContent = `SELECTED: ${['X', 'Y', 'Z', 'T'][stackCursor] || '?'}`;
+        displayEl.classList.remove('display-error');
       } else {
         displayEl.textContent = 'READY';
+        displayEl.classList.remove('display-error');
       }
       modeLabel.textContent = calc.angleMode;
-      errorEl.textContent = '';
       renderKeypad();
     }
 
@@ -773,10 +824,12 @@
           clearStackSelection();
           calc.command(name);
         }
+        currentErrorMessage = '';
         syncInputFromState();
         render();
       } catch (error) {
-        errorEl.textContent = error.message;
+        currentErrorMessage = error.message;
+        render();
       }
     }
 
@@ -960,7 +1013,8 @@
           execute(action.value);
         }
       } catch (error) {
-        errorEl.textContent = error.message;
+        currentErrorMessage = error.message;
+        render();
       }
     }