The "Design Bites" series is about learning or appreciating just one design element of one game. It's about applying an analytical eye, even if it doesn't touch on everything.
Ikaruga is a Japanese arcade bullet hell shmup developed and released by Treasure in Late 2001. It features a color-swapping mechanic where the player can switch between blue and red, making them impervious to bullets of the same color.
When we play games, we often try to "outsmart" them. We look for broken spell combinations, overpowered guns, experience farming tactics and other things to give ourselves the edge.
In Ikaruga, the story is the same: "By staying in blue form forever I can ignore half of the danger on screen! All I need to do is kill off all red enemies."
About 2 minutes later you're presented with a pair of alternate colored enemies that slowly but surely push you to switch, as you are placed in a position where simply dodging one color is impossible. The game doesn't really tell you that you need to switch, it just sort of lets you slowly realise it yourself. Soon after you are pummelled with a wide array of different bullet patterns that require you to switch frequently.
The cool thing about Ikaruga is that it likes to gently dispel notions like these with small challenges before hitting you with the hard stuff. It's a good lesson in tutorial design.
Further encounters teach you to use color switching strategically even when its not necessary to switch, one instance is killing off the small opposite-color enemies in the first boss fight to make switching during the bosses alternating patterns uninhibited.