It's not exactly simple but here's what you can do.
Here is a 5 second duration rectangle. An animation has been added to the end of the Rectangle.
The animation needs to be grabbed and Dragged out to extend it, from the Start to the End of the rectangle.
You can start anywhere you want. I started at the end of the rectangle on the next to last frame. I Changed the color. Then I went back one frame using the "Previous Frame"button. Changed the color again. You have to do that for every frame. There is no short cut for this.
You could make a much shorter animated rectangle. Copy and paste them end to end. That way you save yourself a lot of time.
Just don't have the same colors at the start and end of the animation so it looks good.