Link width of two shapes inside one shape layer

Hello,

I'm trying to design this responsive text box but can't get it quite right.

I have a shape layer containing the following shapes - 
(Group 1) - 

- Color (Rectangle)

- Base (Rectangle)

- Stroke

The "COLOR" shape is a thin highlight at the bottom of the box. I need to link the width of that shape to the width of the "BASE" shape. 

The "BASE" shape is controlled by an expression to make it responsive to a text layer. The main issue I'm having is the "BASE" shape is rounded. I can copy the same sourceRectAtTime expression to the "COLOR" shape but then can't shrink the height using the transfrom property without smooshing the rounded corners. So I'm hoping I can just link the X property in the "COLOR" shapes Size controller to the X property of the "BASE" shape.