Hey, is it possible to do a multivariate test using growthbook? As in multiple variant types (eg, button colour as well as size)

Yes it is

You can have multiple variables per experiment. And you can run multiple simultaneous experiments. But currently we don’t have an analysis that says which combination of variants is the highest performance - usually, however, the winner of test 1 will also be the winner when you look at test 1 and 2.