Breeze icons currently come in two styles: monochrome and colorful.

Generally, the monochrome style is used for small icons: 16px, 22px, and 24px (for compatibility with GTK which uses the 24px size). Colorful icons are generally 32px or larger.

This leads to a general rule: use monochrome for small icons, and color for large icons. This works fine for icons all of whose sizes are in one style or the other.

It stops working so well for icons that have multiple sizes, with small monochrome versions and larger colorful versions, because some of the time we want to use colorful icons for small sizes and some of the time we want to use monochrome icons for large sizes.

For example, we want an app's icon to be colorful for the small 16px version shown in the titlebar, but monochrome if it happens to have a system tray icon (e.g. Plasma Vault). And we want toolbar icons to continue to use the monochrome style at 32px and larger sizes even if they happen to have colorful versions.

Right now we don't really have a way to do this within the confines of a single icon theme.