A single property, tween is not overloaded, but it should accept either a single argument (target value from current) or an array [value1, value2]. Your code is correct in this regard.
For a different approach, you can try using morph instead:
$("element").addEvent("click", function() {
this.morph({
color: ["#A00000", "#99CCFF"]
});
});
Although tween seems to work fine here as well. When you mention that it's "rejected," what exactly do you mean? Does nothing happen? Do you receive an exception? In which browser? Which version of MooTools are you using? Have you used !important in the CSS? This question is too broad to answer without more specific information.
If you want BOTH elements to change color, you can use the following code:
$("element").set("tween", {
onStart: function() {
this.element.addClass("tweening");
},
onComplete: function() {
this.element.removeClass("tweening");
}
}).addEvent("click", function() {
$("element").tween('color', ["#A00000", "#99CCFF"]);
});
Also note:
#element.tweening strong {
color: inherit;
}
This will temporarily disable the styling while you animate.