The red comes from Achiote paste and chile puree. For about 2-3lbs of beef I like to pour boiling water to cover 3-4 guajillo peppers, half a medium onion, and 6-8 cloves of garlic. In a blender, squeeze an Orange and a couple of limes. If you have some pineapple juice on hand add a 1/4 cup. Then add half a brick of achiote paste and a 2-3tsp chipotle powder or a chipotle in adobo. After 20 minutes of soaking, add the peppers and onions. Season with salt, pepper, and Mexican oregano. Blend on high until smooth. Salt the meat and pour the marinade over everything mixing well. Let it sit for a few hours or even overnight if you like.

The Mexican meat markets typically add red food coloring so I can't say how it will compare to what you're used to, but they are delicious.

Thanks Grok

Ingredients (Serves 46)

For the marinade
3-4 dried guajillo chiles, stems and seeds removed
1/2 medium white onion, roughly chopped
6-8 cloves garlic
1 orange, juiced
2 limes, juiced
cup pineapple juice (optional but recommended)
brick achiote paste (about 1.52 oz)
23 tsp chipotle powder or 1 chipotle pepper in adobo
1 tsp Mexican oregano
Salt and freshly ground black pepper, to taste

1. Soften the chiles and aromatics
Place the guajillo chiles, chopped onion, and garlic in a bowl. Pour boiling water over them until fully submerged. Let soak for about 20 minutes, until the chiles are softened and pliable.

2. Build the marinade
In a blender, add:
Orange juice
Lime juice
Pineapple juice (if using)
Achiote paste
Chipotle powder or chipotle in adobo

Add the soaked guajillo chiles, onion, and garlic. Season with Mexican oregano, salt, and black pepper.

Blend on high until completely smooth. The marinade should be thick, deep red, and fragrant.

3. Marinate the beef
Season the beef lightly with salt. Place it in a large bowl or resealable bag and pour the marinade over the meat. Mix well so everything is evenly coated.

Refrigerate for at least 23 hours, or preferably overnight for deeper flavor.

Just go buy your fajita meat from La Azteca.