Your example:

MyObject.prototype.color="red";
var obj1=new MyObject();
MyObject.prototype.color="blue";
MyObject.prototype.soundEffect="boOOOoing!!";
var obj2=new MyObject();

is wrong:
if you change the prototype of an object, all objects that inherit or are instantiated from that object will have the new value.

if you query obj1.color before line 3, you will get ‘red’ as a response. If you query obj1.color after line 3 you will get ‘blue’.