diff --git a/src/main/java/com/mythlane/gravityflip/region/GravityFlipRegion.java b/src/main/java/com/mythlane/gravityflip/region/GravityFlipRegion.java
index c22827f..352dd0d 100644
--- a/src/main/java/com/mythlane/gravityflip/region/GravityFlipRegion.java
+++ b/src/main/java/com/mythlane/gravityflip/region/GravityFlipRegion.java
@@ -29,6 +29,16 @@ import com.hypixel.hytale.math.vector.Vector3d;
  *       NPCs in-region are NOT flipped / seeded.</li>
  *   <li>{@code AffectItems}    — <b>optional</b>, default {@code true}. If {@code false},
  *       item {@code PhysicsValues.invertedGravity} is NOT mutated.</li>
+ *   <li>{@code VisualColor}    — <b>optional</b> (Plan 03-05). Default {@code "#00FFFF"} (cyan).
+ *       Hex string {@code #RRGGBB} rendered as debug cube color. Fallback to cyan on invalid hex
+ *       (parsing + validation lives in {@code RegionVisualizer}, NOT here).</li>
+ *   <li>{@code VisualMode}     — <b>optional</b>, default {@code "Outline"}. One of
+ *       {@code "Outline"} / {@code "Faces"} / {@code "Both"} / {@code "None"}. Unknown value
+ *       falls back to {@code "Outline"}.</li>
+ *   <li>{@code VisualRefreshMs} — <b>optional</b>, default {@code 1000}. Emission period in ms
+ *       for the debug shape (refresh cadence ; TTL = refreshMs * 1.2 to avoid flicker).</li>
+ *   <li>{@code VisualOpacity}  — <b>optional</b>, default {@code 0.5}. Cube opacity in
+ *       {@code [0.0, 1.0]} (clamped in {@code RegionVisualizer}).</li>
  * </ul>
  *
  * <p><b>Back-compat :</b> a legacy {@code regions.json} containing only Name+Box+Enabled
@@ -69,6 +79,15 @@ public final class GravityFlipRegion {
                             (r, v) -> r.affectNpcs = v, r -> r.affectNpcs).add()
                     .append(new KeyedCodec<>("AffectItems", Codec.BOOLEAN),
                             (r, v) -> r.affectItems = v, r -> r.affectItems).add()
+                    // --- Plan 03-05 : 4 optional visualization fields ---
+                    .append(new KeyedCodec<>("VisualColor", Codec.STRING),
+                            (r, v) -> r.visualColor = v, r -> r.visualColor).add()
+                    .append(new KeyedCodec<>("VisualMode", Codec.STRING),
+                            (r, v) -> r.visualMode = v, r -> r.visualMode).add()
+                    .append(new KeyedCodec<>("VisualRefreshMs", Codec.INTEGER),
+                            (r, v) -> r.visualRefreshMs = v, r -> r.visualRefreshMs).add()
+                    .append(new KeyedCodec<>("VisualOpacity", Codec.DOUBLE),
+                            (r, v) -> r.visualOpacity = v, r -> r.visualOpacity).add()
                     .build();
 
     // Package-private mutable fields written directly by the codec setters.
@@ -84,6 +103,12 @@ public final class GravityFlipRegion {
     boolean affectNpcs = true;
     boolean affectItems = true;
 
+    // Plan 03-05 : visualization fields — defaults applied when key absent in BSON.
+    String visualColor = "#00FFFF";
+    String visualMode = "Outline";
+    int visualRefreshMs = 1000;
+    double visualOpacity = 0.5;
+
     public GravityFlipRegion() {}
 
     public GravityFlipRegion(String name, Box box, boolean enabled) {
@@ -118,6 +143,18 @@ public final class GravityFlipRegion {
     public void setAffectNpcs(boolean v) { this.affectNpcs = v; }
     public void setAffectItems(boolean v) { this.affectItems = v; }
 
+    // --- Plan 03-05 getters / setters ---
+
+    public String getVisualColor() { return visualColor; }
+    public String getVisualMode() { return visualMode; }
+    public int getVisualRefreshMs() { return visualRefreshMs; }
+    public double getVisualOpacity() { return visualOpacity; }
+
+    public void setVisualColor(String v) { this.visualColor = v; }
+    public void setVisualMode(String v) { this.visualMode = v; }
+    public void setVisualRefreshMs(int v) { this.visualRefreshMs = v; }
+    public void setVisualOpacity(double v) { this.visualOpacity = v; }
+
     /** Convenience accessor for tick-loop / physics consumers in Phase 02-02. */
     public Box asBox() { return box; }
 }
diff --git a/src/test/java/com/mythlane/gravityflip/region/GravityFlipRegionCodecTest.java b/src/test/java/com/mythlane/gravityflip/region/GravityFlipRegionCodecTest.java
index df797e3..a8e1ff7 100644
--- a/src/test/java/com/mythlane/gravityflip/region/GravityFlipRegionCodecTest.java
+++ b/src/test/java/com/mythlane/gravityflip/region/GravityFlipRegionCodecTest.java
@@ -144,6 +144,41 @@ class GravityFlipRegionCodecTest {
         assertFalse(decoded.isAffectItems());
     }
 
+    // ---------- Plan 03-05 : 4 visualization fields ----------
+
+    @Test
+    void roundTripPreservesVisualFields() {
+        GravityFlipRegion src = baseRegion();
+        src.setVisualColor("#FF8800");
+        src.setVisualMode("Faces");
+        src.setVisualRefreshMs(500);
+        src.setVisualOpacity(0.75);
+
+        GravityFlipRegion decoded = roundTrip(src);
+
+        assertEquals("#FF8800", decoded.getVisualColor());
+        assertEquals("Faces", decoded.getVisualMode());
+        assertEquals(500, decoded.getVisualRefreshMs());
+        assertEquals(0.75, decoded.getVisualOpacity(), 1e-9);
+    }
+
+    @Test
+    void roundTripPreservesVisualDefaultsWhenFieldsAbsent() {
+        // Region construite via constructeur legacy 3-arg — simule un regions.json legacy
+        // (ni 03-04 ni 03-05 présents).
+        GravityFlipRegion src = new GravityFlipRegion(
+                "legacy-viz",
+                new Box(new Vector3d(0, 0, 0), new Vector3d(1, 1, 1)),
+                true);
+
+        GravityFlipRegion decoded = roundTrip(src);
+
+        assertEquals("#00FFFF", decoded.getVisualColor(), "default VisualColor=#00FFFF (cyan)");
+        assertEquals("Outline", decoded.getVisualMode(), "default VisualMode=Outline");
+        assertEquals(1000, decoded.getVisualRefreshMs(), "default VisualRefreshMs=1000");
+        assertEquals(0.5, decoded.getVisualOpacity(), 1e-9, "default VisualOpacity=0.5");
+    }
+
     private static GravityFlipRegion baseRegion() {
         return new GravityFlipRegion(
                 "r",