I'm glad someone brought this up. Rainbow is a term players throw around a lot but there seem to be several meanings to it. And many will claim monogrids are "superior" to rainbow because they don't understand the design or the goals of a rainbow grid.
The first point of this article is to illustrate what a "good" rainbow grid looks like. Note the kamihime are all expected to be of the same element.
This article assumes your endgame goal is to have all 6 grids filled with maxed out SSR weapons. If you plan to only build one grid and ignore all other elements, stop reading this thread. 6 grids have several advantages over 1. You get advantage in every battle. Many event missions also require a specific element so building all 6 allows you to complete those missions.
SSR take a while to acquire so the 2 choices are to fill the remaining slots with SR weapons or SSR from another element. The former grants extra weapon skill buffs but less raw stats and requires you to spend resources. The latter gives higher stats and is completely free since the placeholder weapons are just weapons you use in another grid.
The second point is it shows what a "typical" rainbow looks like for players currently. It demonstrates that a maxed rainbow grid is fairly close to a maxed monogrid in terms of relative strength. And it requires no extra investment in SR which will just get replaced later.
Third, it compares the rainbow to a SL10 monogrid which is roughly what you'd expect to make using the same resources as a rainbow. This demonstrates that it's better to focus on your SSR and only use extra resources for SR. It also shows that it's a bad idea to split your focus and try to build all your weapons simultaneously.