They put all the buttercream colors on top of a plastic (example: plastic film, an open bag ...), then roll it up to a straw and add it to the pastry bag. Then you just have to confection the way you want, the colors do not mix and form a beautiful rainbow. I'm sorry, with you don't understand, I'm not very good at english ╮(￣▽￣)╭
So with you want to know more, you can go to this site (but it's in portuguese) (=・ω・=)
https://poplembrancinhas.com.br/como-fazer-cupcake-arco-iris/
Made separately then added into the same icing bag in layers or stripes then ta-da! Rainbow icing
Thank you very much. That explains it. (๑•ㅂ•)و✧ Even though the webpage is in Portuguese the pictures are self-explanatory.
It's rare to see someone who's willing to switch or even compromise in BL and even rarer in such a gentle manner. Most of the time it's 'hey I'm top, you're bottom, wham bam'. No question asked.
I always felt that overly colored cake looks unappetizing and inedible.
(〜￣△￣)〜
to Hikaru for actually asking his partner, what he wants to do. Cute story. And that cake, I know how to make the sponge cake coloured, but how was the cream made to be a rainbow without mixing?