How can I change color of actor by sec?

Well, I want to change color of some actor by time like gradation.

0 sec: white > 0.1 sec: gray but almost white > 0.2 sec: gray but almost white and graier than before > … > 1 sec: black
Like this.

But I can’t find the way how can I change like this.

So you want simple blending from white to black?

You can use sine function, linear sine, or even simple Frac(Time).