Need help with progress bar shader

Hi. I am trying to have it so when a player touches a computer, the bar fills with red starting at the left until it has hit the right side. I understand the logic behind it but am wondering what way you would recommend I go about doing this with a shader or if anyone can recommend something. For instance. here is the texture we are using:

I have looked at a bunch of shaders and couldn't figure out how they would actually apply to my situation where I just want red to fill the bar.

Can anyone explain if this shader would help me accomplish this or if they have something else that would work?

http://wiki.unity3d.com/index.php?title=Reveal_Non-Uniform_Texture

Furthermore, would it just make sense to calculate how far out the red should go based on the time the use has held the screen (a number I already have been calculating) and then if the pixel is between the y range and the x start and the x end, make it red.

What would you recommend?