Favorite clips that are pre-keyed with alpha channel...

No - if you are going to key it, then offer it keyed. (with some exceptions - clouds for example) - but green or blue are actually the best key colors - white is ok, but in photo work only - the objective is to have a key background color that is the furthest from any skin tone and/or clothing. Basically - a background color to be keyed should not appear on the subject - otherwise you will need to do rot-scoping - a whole other nightmare. :)

Making "run of the mill" cloud time lapse footage more useful and diverse:



https://www.pond5.com/stock-footage/841266https://www.pond5.com/stock-footage/841262https://www.pond5.com/stock-footage/841253https://www.pond5.com/stock-footage/841246https://www.pond5.com/stock-footage/841236