Animating a change in color of type from bottom up

Hi, All...
Pardon me if this is a rudimentary question...I'm trying to do something that seems simple, but I haven't been able to figure it out. Here's goes:

In a title sequence for a trailer I'm working on, I have two animated elements in play and I want them to interact with each other.

-- There's a line of red text created in Motion's type tool using Helvetica...tracking animation slowly separates the space in between the letters
-- As this is happening, a red rectangle (same color as the text), that was created using Motion's rectangle tool, pushes up from the bottom of the frame and intersects with the type.
-- Since the color of the type and rectangle are the same, I want to make the type change color to white as the rectangle intersects it...so the type would essentially turn white from the bottom up as though the red rectangle was erasing the color from the text as it intersected.

Any ideas on a quick and easy way to make this happen? I'm sure there's something I could do using a mask of some sort, but as a Motion newbie, I can't intuitively figure it out. The key thing is that the white gets kind of painted on to the text from the bottom up, rather than all at once.

Thanks in advance for any help anyone can offer!