I love the color palettes, and I really like the sprites! They feel like a really neat in-between of galaxian and space invaders.
In terms of gameplay, I think the way you made the enemies shoot is very cool. Picking a random enemy to shoot every second or so is a neat way of increasing the difficulty as you clear a stage, as the last enemy you take out will be firing constantly.
In terms of criticism, I think the lowest row of enemies is a bit too low on the screen. Shots fired from that row go from being fired to killing the player in a matter of only a few frames, and it feels a bit cheap some times.
Additionally, I think it's missing a certain energy in the moment-to-moment gameplay. This is partially due to the fact that none of the enemy attacks are targeted towards the player. As such, the only time I'm in danger is when I put myself in danger, which means most players are going to go about clearing the level in a slow, methodical way. I think it would feel a lot more tense and satisfying if there were another constant threat to keep the player on their toes. This could even just be a time limit, so the player has to focus on clearing the enemies quickly in each level. It's a rule of thumb in game design that players will tend to interact with your game in the least interesting way possible (because that's usually the safest route to victory) so it's important to notice when that happens and force them to engage with the systems head on.
All in all, great job on your game! It's already pretty solid, and with a couple tweaks I think it'll be elevated to the next level.