I'm trying to do that but it doesn't work. This is the fragment code i tried:

"precision mediump float;" \
"uniform vec4 a_Color; bool on; " \
"void main() {gl_FragColor = on == true ? a_Color : vec4(0,0,0,0); on = !on;}"

However, seems that "bool" is not "global', and thus, nothing is drawn at all. I also tried to use position.x % 2 == 0, but my glsl language is 1.0 and thus does not support it.