One style. 24 px grid, 1.5 px stroke, rounded ends and joins, no fill. Ideas sit in a gradient tile.
Scale in steps: 16, 24, 32, 48 px. Below 20 px, a 1.8 px stroke reads better.
Ink by default. Red marks the one icon that matters on a screen; blue is for engineering data.
On the web, use the sprite: <svg><use href="sprite.svg#sg-flag"/></svg>.