Hi Nick, the construction line color is in the moi.ini file, construction lines use the same color assigned to the "straight snap" lines (the dashed lines that show up when straight snap is active like when you move near the x or y axis to make a drawn line go in that same x or y axis direction).

Look for the StraightSnapLineColor entry under the [Drawing Aids] section:
StraightSnapLineColor=103,0,0,0

That's made up of 4 values separated by commas, each value goes from 0 to 255 - the first one is transparency and the 2nd, 3rd, and 4th ones are Red, Green , Blue.

So for example to make construction lines appear as blue it would be:

StraightSnapLineColor=103,0,0,255

- Michael