Ok I understand your concern and thx for a very detailed explanation. I still plan to use rounded corners for my design but maybe there's a workaround for that e.g. by not using the whole canvas size and filling up the remaining space by one of those 3 colours.

Thank you for your feedback. This seems a great idea on paper. However, from my previous experiences this would result only in confusion and possible loopholes for simply using transparency as another colour to use inside. Defining how much would still count as rounding the edges or adding holes in the paper and when it would start to become cheating would make too long and probably could never completely predict all possibilities. Using it as a 4th colour could be alright but it is unpredictable since backgrounds are different. And if so, then why just simply not add another colour? Becasue the discussion would start again about adding transparency, replacing it for a colour for consistency and then having 5 colours and so on... Sometimes simplicity is really the best solution. This is a challenge about a design project not about photorealistic paper, that's why there are only 3 colours. But you can still be creative within those boundaries. Mark the rounded edges with black, etc. 

Can we please make transparency optional in this one? Card corners are typically rounded so I think the transparent color used in the corners would make them look right. At least it works for my design :) Or perhaps if someone wants to have torn edges or holes in their cards it may be useful feature as well :) Thanks!