You have a marvellous and addicting game here but there are some critical game design flaws that are really holding back this game .
Make walls and instant death objects obvious. Iv come across orange walls, thinking they would give me a boost, only to die instantly to them. In games like this colour is important, you have let the player know Blue, Green and Orange are good colours that give positive effects, dont go confusing them by making it kill you. This would not be so bad if it was an attempt to fool the player, but the fact that colour walls were not added into the tutorial makes it something that will kill the player the first time playing. it would be best to make walls one type of colour or have them themed per level. having the colour indicate the shape is fairly wasted, the load distance is not so far that this would make a difference.
enemy bullets! out of everything in the game enemy bullets are the smallest and have the lowest contrast compared to everything else. they are difficult to see and sometimes even the hardest things to dodge. bullets, particularly those which do so much damage need to be big if not incredibly bright and stand out the most. The player cant dodge them if they can barely see them next to the bright and flashing enemies on the screen.
While knowing how many points and enemy nets you, when an enemy dies their explosion + the points value display tends to blind the player in that direction, easily brighting out and outright blocking the visibility of enemy bullets or the blocks behind them.....maybe make the deaths less flashy?
Shield is nearly a completely useless power up. its rare and only lasts for a handful of seconds, instead it should act much like the weapon power up in bars, additionally it should not expire on its own and should instead only disappear when an appropriate amount of damage it taken. if not, allow the player the choice in its use. As mentioned, give the player a shield bar of how many charges it can hold, on a press of a button a shield is activated for either a time period or until enough damage is taken before it dissipates. As they stand now, they could be removed from the game with little influence on the gameplay.
It maybe just me but the hitbox seems a little large at times. in a game like this, particularly due to the speed, its best to have the hitbox slightly smaller than what the player expects, this gives them a little more wiggle room and makes small mistakes a little more forgiving. not to say the game should be forgiving, but due to the short load distance and constantly increasing speed, small mistakes should not be punishable to instant death.
I rarely login or comment....but this game is so close to being a gem I couldnt help it. many games are like this, but few get it so close to getting it right as this one has!