_randomColor (base, spread) {
		const v = new THREE.Color();

		v.copy(base);

		v.r += (Math.random() * spread.x) - (spread.x / 2);
		v.g += (Math.random() * spread.y) - (spread.y / 2);
		v.b += (Math.random() * spread.z) - (spread.z / 2);

		v.r = Math.max(0, Math.min(v.r, 1));
		v.g = Math.max(0, Math.min(v.g, 1));
		v.b = Math.max(0, Math.min(v.b, 1));

		return v;
	}
Create a new THREE.Color instance and given a base vector and 
spread range vector, assign random values.

Note that THREE.Color RGB values are in the range of 0 - 1, not 0 - 255.

@private

@param  {THREE.Vector3} base
@param  {THREE.Vector3} spread
@return {THREE.Color}