The idea is pretty great, I love the idea of mixing magic and how 3 colors can be used effectively. It is a very elegant mechanic. The isometric view is one you don't see enough even among indies these days! Glad you could manage to make what felt like a vertical slice of a more developed product.
Also the art for the banner is great, honestly something I would see in a finished product.
I feel that due to every attack in the game having a set up time, both in selecting colors, and from the moment the spell is, let's say, summoned, that makes the gameplay a bit slower than I would have expected.
Also another complain would be regarding the fact WASD and the numbers require the same hand, I get the idea of keeping them close due to the fact one hand will invariably be in the mouse at all times. It is an issue that FPS games also struggle with, but I would argue moving while picking colors is more important here than moving and changing weapons at the same time in an FPS game, or selecting skills in a MOBA. If enemies didn't also give a comfortable time for dodging, it would have been an issue. Even if they didn't, maybe right mouse button as a dodge or shield would have helped the design anyway.
Honestly if you have the desire of developing this further, I would personally suggest maybe taking inspiration on MOBAs and Diablo-likes, instead of the set up time for spells I would make them faster, allow right mouse button for dodge, mouse position for aim, either keep aims like right now or fixate the area for spells in front of the character and mouse defining a position, or even a mix of both in different skills. I would also suggest maybe ZXC for spells instead of 123, or at least redundant with 123, because it doesn't require moving one's wrist while taking fingers off the WASD keys, which speeds up and lowers strain on hand for keys, since that is your basic attack through and through. Maybe the colored system isn't used on all attacks, but in key attacks and skills or barrier breaks.
Anyway just rambling at this point, whenever an entry inspired me to think things like that it just means it was pretty inspiring! Great job!