The oven gets you darker with far less risk of losing it
Making the roux in the oven (dry-bake or oven-braise method) instead of stovetop stirring reliably reaches deep, dark color (7-8/10) with less risk of scorching than active stovetop stirring.
A stovetop roux is an hour of continuous attention in which one lapse ends the batch — the pan is hotter than the flour wants and the only thing between you and black specks is stirring. An oven surrounds it with even heat instead, so it browns on its own schedule and forgives you for answering the door. It is the same chemistry arriving with a much wider margin.
What to do: 300–350°F, stirred every twenty minutes or so, one to two hours. Or bake the flour dry with no fat at all and whisk the oil in afterwards, which two cooks rated 9 out of 10 for colour and is the fastest route of any.