The structure of rainbow charts encourage you to test risky assumptions early, allowing you to move forward and fast in the right direction. By listing out all your assumptions, you can be deliberate and prioritize testing the riskiest assumptions to avoid massive mistakes.
Icons are huge part of making a base line grid work correctly across a user interface. There is a lot of tricky math to get this perfect, but if done correctly the integrity of your icon will maintain in the spectrum of screen densities (Retina, 1x, 2x, MDPI–XXXHDPI…if this doesn’t make sense read this). They key to executing icons in this manner is to ensure that all shapes, strokes, and margins are “even” (2, 4, 6, 8), so that you are left with a centered icon in the 24 x 24pt area that can also be multiplied (x2, x3, x4) and scale evenly.