Most of us first learn from neobodybuilding magazines and program solely based on muscle groups (color), but it's impossible to categorize exercises and program effectively solely from a muscular stress perspective.
Consider the color to be a facet of local stress.
Imagine a blacked out human body. Now imagine certain muscles and structures lighting up upon moving. You can see where things are happening locally.
But there's also global stress. Every exercise affects the entire organism. Even though an exercise is sometimes classified as “upper” or “lower” or “press” or “pull” there’s a generalized systemic stress with every exercise.
(To read more about the global effect, see this chunk series.)
Here are some of the factors that influence the global stress:
Used percentage of maximum ability (% 1RM). The closer to your max you train, the more global the stress gets.
Number of muscles involved. The more muscles that are involved, the more global the stress gets. So a deadlift (in which your legs, back, and grip are heavily taxed) is much much much more stressful than a bicep curl.
Body position (standing, seated). Standing exercises are typically more stressful because there’s more muscle mass involved. Takes a lot of energy to stabilize your body in space.
Total weight lifted. A bench press can be more taxing than an overhead press even though it’s done lying down simply because you can press a whole lot more weight during the bench press.
Specific muscles involved. The hand and feet are neurologically intensive areas. If you do a lot of intense grip work, your system will be more fatigued than if you could have gotten away without using your hands. (Think of a deadlift with straps vs. a deadlift without straps.)
Level of psycho-physiological arousal. Bashing your head against the wall and listening to Trivium will zap you more than being relaxed and listening to classical music. DON’T LET THE RESIVOIR DOGS KNOW YOU’RE LIFTING.
Complexity of movement. Complex movements are more stressful. More joints, more muscle mass. But also more mental energy (to fathom the complexity).
Novelty. New things are more stressful than old things. You get better dealing with a stressor over time. The total-body-freak-out global response from any one stressor lessens with repeated exposure. The body learns how to cope. It quarantines the global effect, dealing with it on a more local level. So familiar adapted to stressors aren’t as alarming.
In general, the best way to think about it:
A) How close am I to moving my body through space in a way that opposes gravity? The more supergravity the conditions, and the more holistic the conditions (spread across your body), the bigger the general stressor.
B) How prepared am I for the stress? Deadlifts are more taxing than bench presses, but if I've been deadlifting every day for ten years and I never bench pressed in my life, it'll take a bite out of me.
But, in general, there's always a difference between a head cold and a fever. Just as there’s always a difference between a barbell biceps curl and a barbell deadlift.
Programming is a delicate balance between picking the right exercises and managing overall stress load.
Deadlifts deliver a bigger global hit, but they don’t hit the biceps like curls do. So if you want to maximize biceps stress, it doesn't make sense to deadlift.
But, at the same time, doing just a biceps curl might not a great idea because the global hit isn't all that large.