It probably has to do with replacing gl_FragColor.rgb *= vec3( 1., 0., 1. ); in the last step of the fragment shader, but I can't figure out a way that:

a) works with any structure of shader - probably requires some interesting regExp
b) won't interfere with the shader source update mechanism (how to distinguish user-type changs from that change) - may be creating a parallel shader for that?