Colour Grating Shader, go to http://editor.thebookofshaders.com and paste code this in. Or go to http://glslsandbox.com/e#56894.0

colorgrating.frag

//red-green sinusoidal grating
#ifdef GL_ES
precision mediump float;
#endif

uniform vec2 u_resolution;

float contrast = 0.75;

vec3 colorA = vec3(1.0,0.0,0.0);//red
vec3 colorB = vec3(0.0,1.0,0.0);//green
vec3 baseColor = vec3(0.5,0.5,0.5);//base color is grey

void main() {
    vec2 st = gl_FragCoord.xy/u_resolution;
    vec3 color = vec3(0.0);

    float pct = (sin(st.x*30.0)+1.0) / 2.0;
    
    //uncomment this line to make a nice smooth square wave grating
    //pct = smoothstep(0.49,0.51,pct);
    
    //we first blend our colours from base using 0-1 contrast range
	if (contrast < 1.0) {
		colorA = mix(baseColor, colorA, contrast);
		colorB = mix(baseColor, colorB, contrast);
	}

    // Mix uses pct (a value from 0-1) to
    // mix the two colors
    color = mix(colorA, colorB, pct);

    gl_FragColor = vec4(color,1.0);
}