fadetocolor.glsl

#ifdef GL_ES
precision highp float;
#endif

// General parameters
uniform sampler2D from;
uniform sampler2D to;
uniform float progress;
uniform vec2 resolution;

uniform vec3 color;
uniform float colorPhase; // if 0.0, there is no black phase, if 0.9, the black phase is very important

void main() {
  vec2 p = gl_FragCoord.xy / resolution.xy;
  gl_FragColor = mix(mix(vec4(color, 1.0), texture2D(from, p), smoothstep(1.0-colorPhase, 0.0, progress)),
                 mix(vec4(color, 1.0), texture2D(to,   p), smoothstep(    colorPhase, 1.0, progress)), progress);
}

uniforms.default.json

{ "color": [0,0,0], "colorPhase": 0.5 }