Selecting multiple objects with cryptomatte

Hi,

In your case You can add a plate in the background. Then select it and use an other effect to negativ the matte.

Otherwise I don't have the solution to your idea

Expressions can't do anything you can't do with keyframes. Therefore your idea has no merit. Other than that "Cryptomattes" are just spiced up versions of RGB Mattes and they are treated the same - isolate the channels e.g. using Set Channels/ Shift Channels, apply blending operations and adjustment effects to them to mutually add, divide, subtract and multiply. If you look up old tutorials on such stuff, you'll understand. Otherwise it seems to me that your workflow simply doesn't make much sense. It would likely be more productive if you actually assigned proper object IDs to your 3D objects and rendered those as passes or create a custom particle output e.g. based on proximity, camera depth and so on. Relying on automatic polygon/ object/ material IDs such as an untamed and unmanged cryptomatte produces can only get you so far and is simply impractical fur such small objects in huge numbers.

Mylenium