Simple DIY rainbow magic with Skittles candies. Form a circle with Skittles on a plate (colours should be in repeated order, preferably according to colours of the rainbow e.g. purple, green, yellow, orange, red), then pour hot water over them. Wait for the magic to unfold right in front of your eyes
Also funny are the various YouTubers attempting to replicate the effect only to end up with a brownish mix of melted candy slime on their plate.