I don’t punish as such at that age but there are immediate consequences which are relevant to what they are doing. I also try and remove the temptation for mischief before they think of doing it themself. I also don’t react
So for pushing the plate off the table, to be honest I would try and remove it as soon as it’s obvious they’ve lost interest in eating. Failing that, I would get a plate that sticks to the table. If the food is pieces rather than messy food I would also get them to help clear it up and just keep repeating ‘no playing until it is picked up’. I find boring them into submission can work really well.
i don’t think mixing the playdoh colours together is undesirable behaviour as such, although it’s understandably frustrating to you. It’s natural experimentation at that age. If you don’t want them to do it then just give them one colour or give them two colours and then give a warning then remove as soon as they start mixing it.
I will always warn twice and then do what I’ve threatened, which is usually remove the item they are misusing, or remove them from the situation. I speak clearly and down on their level and I never shout. I don’t need to because they know that I mean what I say and that if I say that I’m going to take something away then I will 100% do it. You do have to be realistic with expectations though. Children that age like to push boundaries and they love to get a reaction so being calm, firm and consistent does make a difference.
There isn’t a magic bullet that works all the time. It’s a process of steady socialisation to gradually turn them into reasonable, non criminal human beings. The behaviour that you model will shape how they behave in the future - so if you shout and make threats that you don’t carry out to stop unwanted behaviour then they will think that shouting is a normal way to behave.
Ages 2-3.5 are brutal. They’re developing a lot of free will and personality with absolutely no sense of safety or moral compass to moderate it with. It doesn’t necessarily always need fixing, sometimes it just needs a bit of understanding that it’s just a phase of their development and they (and you) will get through it.