That roux looks a little light, but I'm sure it was still delicious. I think it's hard to take a picture sometimes and get a accurate picture of how dark the roux really is. Not burning it is extremely important so I'd rather take it off a shade lighter than risk burning. Usually I take mine to looking like milk chocolate rather than dark chocolate. I then cook my trinity in it and that tends to take it to a darker brown color. That darker color could be the sugars coming out of the veggies. Who knows? The trinity will quickly cool down the roux initially.
I was born in Beaumont, Tx and my dad's family got there from Louisiana. What I've experienced is probably different than what's typical in Louisiana. My dad cooked gumbo differently from his mother and I cook it differently than him.
It might sound silly but what I've done lately is use a
Thermopen to get an idea of how hot the oil is. It's completely unnecessary but helpful. I like to get the temp of the oil around 380 if it starts getting closer to 400 I'll drop the burner down to low to settle it down. I'll start medium high but I'll drop the temp down to medium later on. I never stop stirring. It takes me around 45 minutes. If you get the oil hotter you can probably cook it quicker, but I cook gumbo maybe twice a year. I'm not risking it. There's nothing worse than burnt roux.
One thing that I think is almost as important as the roux is using a good stock. I make my own stock a few times a year. It freezes really well.