Explanations

Now let’s blend colors.

#define PROCESSING_COLOR_SHADER

uniform vec2 resolution;

void main( void ) {

	vec2 position = gl_FragCoord.xy / resolution.xy;
	
  float v = position.x;

Let’s choose two colors : red and yellow

  vec3 colorA = vec3(1.0, 0.0, 0.0);
  vec3 colorB = vec3(1.0, 1.0, 0.0);

We then mix them up with our gradient value

  vec3 color = mix(colorA, colorB, v);

And set it as the screen color (again, with full opcacity)

	gl_FragColor = vec4(color, 1.0);
}

As expected, we have a nice gradient from red (left) to yellow (right) throw Orange at middle screen.

Full Code Source

#define PROCESSING_COLOR_SHADER

uniform vec2 resolution;

void main( void ) {

	vec2 position = gl_FragCoord.xy / resolution.xy;
	
  float v = position.x;

  vec3 colorA = vec3(1.0, 0.0, 0.0);
  vec3 colorB = vec3(1.0, 1.0, 0.0);

  vec3 color = mix(colorA, colorB, v);

	gl_FragColor = vec4(color, 1.0);
}