すごく簡単な事に揉め過ぎだな
Image.colorにあわせてサンプル、これで動くはずだから終わりにしろ

public Canvas canvas;

void Start () {
Image hoge = null;
foreach (Transform child in canvas.transform){
if(child.name == "MainImage"){
hoge = child.gameObject.GetComponent<Image>();
hoge.color = new Color(0, 0, 0, 1);
} else if (child.name == "SubImage") {
hoge = child.gameObject.GetComponent<Image>();
hoge.color = new Color(1, 1, 1, 1);
}
}
}