Salsa roja is the red one, and in most kitchens that means a salsa built on red chiles, usually dried, sometimes fresh, often with roasted tomato. The defining axis is the chile base: dried guajillo, árbol, chile de árbol, pasilla, or ancho rehydrated and blended, which is what separates it from the tomatillo-driven green. That base sets everything. Dried red chiles bring a deep, slightly bitter, leathery fruitiness and a warm earthiness that fresh chiles do not, plus a brick-red color and a rounder, more roasted heat. Its job on a torta, a guajolota, a pambazo, or a plate of tacos is to add concentrated chile depth and a measured burn that pushes against fat and starch; it is the dark, savory red note, not a bright fresh one.
The make is where it lives or dies. Dried chiles are wiped clean, often toasted briefly on a comal until fragrant but not scorched, then soaked soft and blended with garlic, onion, a little of the soaking liquid, and frequently roasted or boiled tomato for body. Many cooks then fry the purée in a film of hot fat, salsa frita, to deepen and round it. The acid and salt come in last, balanced against the chile so the heat reads as warmth rather than raw sting. A good salsa roja is glossy, cohesive, and layered: chile first, then a savory roasted base, then a clean finish. Poor versions show their shortcuts. Chiles toasted too hard turn acrid and bitter; chiles not soaked enough leave gritty skins and a thin, harsh blend; skip the salt-and-acid balance and it is just heat with no shape. On a pambazo the guajillo version does double duty, the bread itself is dipped in it before frying, so the salsa has to hold up to soaking and heat without going flat.
The chile chosen swings it widely: árbol for sharp upfront heat, guajillo for sweet fruit and color, pasilla for dark raisin depth, and roasted versus boiled tomato changes the body again. The full split between this dried-chile red and the raw-tomatillo green is a real structural fork in Mexican salsa, and that comparison deserves its own article rather than being crowded in here.