Preheat the oven to 400°F (204°C). Line a baking sheet with parchment paper
Toss the sweet potatoes and beets in a large bowl with the oil, garlic, salt, and pepper to coat. Transfer to the prepared baking sheet and roast, tossing after the first 20 minutes, until the sweet potatoes are tender and browned, about 40 minutes. Let cool completely.
Make the dressing
Whisk the orange juice, tahini, vinegar, garlic, 1/2 teaspoon salt, and saffron, if using, in a bowl to blend. Gradually whisk in the oil until emulsified. Season to taste with salt and pepper.
Assemble the salad
Toss the kale, spinach, and the cucumber and bell pepper, if using, in a large bowl with enough of the dressing to coat. Add the roasted sweet potatoes and beets and toss again, adding more dressing if necessary. Transfer the salad to a platter. Garnish with the almonds and serve.