First cut of pulling out the relevant parts of my original game to make a little framework.

assets/shaders/ui_error.frag

@@ -0,0 +1,18 @@
+uniform vec2 u_resolution;
+uniform vec2 u_mouse;
+uniform float u_duration;
+uniform float u_time;
+uniform float u_time_end;
+uniform sampler2D texture;
+uniform bool is_shape;
+
+void main() {
+  if(is_shape) {
+    vec4 color = vec4(1.0, 0.0, 0.0, 1.0);
+    gl_FragColor = gl_Color * color;
+  } else {
+    vec4 pixel = texture2D(texture, gl_TexCoord[0].xy);
+    vec4 color = vec4(1.0, 0.0, 0.0, 1.0);
+    gl_FragColor = gl_Color * color * pixel;
+  }
+}