I’d pick a deep navy blue—almost black, but with a calm, stable vibe. In color theory, blue is the “grounding” hue; the deeper you go, the more it feels trustworthy and serious. Plus, when you mix ultramarine with a touch of cadmium or phthalo blue and a little burnt umber, you get that matte, professional look that makes any sketch feel solid. Just keep the saturation low so it doesn’t startle the viewer—subtlety is key.