Expression to manipulate pixels in image

Position is an array of up to 3 values with no real maximum value

Composition or layer size is an array of 2 values with a maximum value of 30,000

Color is an array of 4 values with a practical range of zero to 1.

This expression would change the fill color of a shape from red to blue as it moved from the top of a composition to the bottom and add or subtract green as it moved from left to right.

c = position[1];
t = position[0];
rbRatio = c / thisComp.height;
g = t / thisComp.width;
r = 1 - rbRatio;
b = rbRatio;
[r, g, b, 1]

The hardest part would be figuring out what color you wanted at what position, this could be simplified if you used some if statements and linear methods to face from one color to another as you approach a known position. You can pull the color values directly from the color picker if you set the Info Panel to display decimal values for color.

You could also tie position to the angles in CC Color Offset or drive any color control that has numeric values like Change Color.

Long and short: No. Expressions modulate property streams, nothing more. Only effects can manipulate pixel buffers, ehich in itself could of course be an answer, meaning perhaps e.g. a basic Fill effect could be rigged to fill a bunch of pixels. It's just not clear what you actualyl want to achieve and a more detailed explanation would be required rather than a generic "Can I...?" question.

Mylenium