Draw random lines on a grid and fill with color

Hi,

I am trying to build a project where player moves and draw lines randomly on a grid.which can create rectangles. I want to fill the rectangle with color or texture when the player completes the rectangle. How do I measure that all four edges of rectangle are drawn to fill the color? Its kind of a effect like a Sega Zoom game.

I am using arrow keys to control player movements. and trail renderer to draw lines.

Please help me to resolve this.