docs: Add missing u_color documentation

site/docs/04-graphics/4.2-material.mdx

@@ -139,6 +139,8 @@ var waterMaterial = game.graphicsContext.createMaterial({
 
 Materials come with a lot of built in uniforms you can use!
 
+* `vec4 u_color` - This is the current color of the material
+
 * `float u_time_ms` - This is `performance.now()` a ms timestamp that starts from initial navigation to the page
 
 * `float u_opacity` - Current opacity of your graphic, [[Graphic.opacity]]

src/engine/Graphics/Context/material.ts

@@ -155,6 +155,14 @@ export class Material {
     this._initialized = true;
   }
 
+  get color(): Color {
+    return this._color;
+  }
+
+  set color(c: Color) {
+    this._color = c;
+  }
+
   get name() {
     return this._name ?? 'anonymous material';
   }