I played the game for about 30 minutes and noticed some small things that could be changed:
1. There are no animations for switching between weapons. A simple rise and lower for the weapons moving in and out would be better than them popping in.
2. There is no way that I saw to know what weapons you have when you first start playing. Maybe show a list of weapons and what keys they belong to at the start of the first wave.
3. The elements being opposite for the enemy color is not explained except through the damage difference. Might want to notify the player after they switch elements what the effect is.
4. It can be hard to tell how much of an effect you have on an enemy with each weapon. Floating damage numbers after a hit could give a lot of feedback to the player on weapon effectiveness.
5. Because the enemies move so slowly it feels pointless that I can gain so much speed from hopping around. An improvement to this might be introducing wall running or wall bouncing. Another choice could be to create a new enemy type that moves faster and can also jump.
To be honest I think the game is already in a really good state. The visual presentation (UI, graphics, effects, environment) looks great and the game play works well. I'm sure you have many more things to add to the game so if you need another review later on I am always free to take another look.