Select any Color on the Color Palette to set the Fill of the Rectangle.
Move your cursor over the Drop Shadow Tool on the Toolbox.
Click the Black Arrow on the Right of this Icon to open a Flyout Menu.
Click on the Rectangle and Drag towards the Center, then release.
An Inside Contour with Linear Colors will be applied to the Shape.
Moving towards the Center creates more Steps for the Inside Contour.
Here's a Contour with decreased Offset.
Adjust the Contour Color Progression by clicking its Icon on the Property Bar.
Clicking this Icon will open a drop down menu.
|Contour Color Progression|
|Clockwise Contour Colors|
|Counterclockwise Contour Colors|