From my experience and from what I've read, it's all dependent on what the tree gives, and that is influenced by temperature, period of the seaon, sap quality , etc. I've read where a long low boil supposedly brings a darker color, but my first batch (see above) was a long simmering cook, (and came out light) and this batch was over gas,rapid boil, and came out darker.
There was a pic over on the maple forum, where a producer had batches from early to late and they got progressively darker through the season. Was an impressive pic.
I focus more on the taste, and the color is a bonus.
As the syrup sits on the shelf though, it seems to darken over time. So, I take what I get.