I go with 1lb DME per gallon of water for a gravity of around 1.050, not sure what a cup ends up being, the only times I've noticed the wort being darker was when I used amber instead of light DME. If the DME is old, it's oxidized a bit (totally fine for a starter) which will tend to darken it a bit. If you had the flame on super high and was doing a *really* vigorous boil for at least an hour (I only go 15 minutes to sanitize) then possibly some maillard reactions. Other than that, either too much DME for the volume of water, serious boil off, I can't think of any way the wort could end up being darker.
Even then, I wouldn't worry about it. I usually chill the starter to put the yeasties to sleep, and decant off the excess liquid. Even if you're doing a pils or something, i can't imagine the residual color from the yeast contributing that much to the final beer.