Testing
As part of the design process you should test your emoji in a range of contexts
As the emojis will often be displayed at a small size it is worthwhile to see how it looks at a range of sizes. Zoom out and see if you (or ideally someone else who doesn't know what the emoji is) can discern what it is at a super small size, such as the size it may be when posted in a chat.
Mutant remix emojis can be used in a wide range of contexts and, as such should be tested on a range of background colours. If you have applied the section on Lines & Outlines correctly then it should be properly legible.
