A Roux's Color Trades Thickening Power for Flavor
A roux is just flour cooked in fat, and how long you cook it changes both its color and what it does: a pale, barely-cooked white or blond roux has the most raw thickening power because its starch is least broken down, while a dark brown roux (used in gumbo) has much of that starch already broken down by the heat, so it thickens far less but contributes a deep, nutty, roasted flavor instead. Recipes that want maximum thickening use a light roux; ones chasing flavor over thickness use a dark one and compensate with more flour or a longer cook.
Sources & methodology: These tips draw on established food-science references —
Harold McGee's food-science writing, America's Test Kitchen /
Cook's Illustrated testing, Serious Eats'
technique reporting, USDA food safety guidance,
and peer-reviewed food-safety research (including UC Davis
cutting-board studies) — not reworded recipe-blog consensus.