You realize color theory is bullshit right?
Color is a gradient of wavelength. It does not arise from 3 primary colors. Realize that those three colors are picked arbitrarily and placed in some technical framework called "color theory." Color theory is a highly inappropriate term for this as it implies the concept is fundamental rather then arbitrary.
Color theory arose from the observation that mixing those 3 colors got you other colors but they didn't realize that the 3 colors were actually arbitrary and that there are many sets of 3 colors that can produce the SAME effect. It's an archaic concept that's still taught in art because of tradition and not because of any science or logic.
The human eye does, however, utilize 3 primary cones to do color sensing. These colors, however, are not the primary colors: Red yellow and blue. It's RGB. Your screen uses RGB because engineers who came up with the concept of RGB weren't so full of it.
For that reason the web designer you hire SHOULD technically be using RGB and NOT color theory. Color theory is literal BS that I feel many designers buy into the bullshit. It makes them sound like they know what they're talking about when really all it comes down to is whether or not something looks good or not.
Additionally, color theory is a TRIVIAL concept. It's EASY to learn and apply. There's nothing advanced here that you can't look up on the internet. You don't need to hire a designer if you want to miss-apply some bullshit called color theory to your "designs". Just make sure you ask your gut whether the design looks good or not because that's actually the thing that needs to be studied here. If any theory needs to be formulated around design and color it needs to be centered around biology/culture to determine what we think "looks good".