I thinks
specific questions would work well. To use the bad boss example:
> I like it! Really good. Maybe just a darker shade of blue there, and change the word ‘giant’ to ‘huge’. Other than that, it’s great!
Let's change this to:
> I like it! Really good. Can you briefly work me through the design process behind it? I'm particularly interested in how you settled on the colours over there, and your ideas behind the word 'giant'.
(This honestly still needs a bit of work, but is getting there)
In this phrasing we show interest in the designer's work, respect their choices and don't assume our ideas are better than theirs. Instead we assume they know what they are doing and give them the space to convince us and also tell us what parts of the design do and do not matter to them, before engaging in giving proper feedback.
The big issue with the cited bad example is that the boss doesn't say why the design needs a darker shade of blue, or why the word ‘giant’ has to change to ‘huge’. It doesn't give any other feedback than "this is wrong, this is the fix". It's not open for debate.