While doing research for this topic, I noticed there is a lot of different advice around. Some people tell you to sugarcoat your constructive criticism with compliments. Others like to be straight-forward and objective. I'm in the second camp, which some people perceive as blunt. And honestly, I can be blunt sometimes. I do prefer to be direct and straightforward when delivering feedback.
I don't think labeling feedback as negative or positive is very helpful. It's more useful to separate the emotional side from the equation. However, as we're dealing with human beings, who have feelings, that's often impossible. So how should we deal with that?
A great theory I found is plussing. It's mostly about signal words. “But” or “need to” are rather negative phrases. Replacing it by words or phrases like “and” or “what if” instead, allows you to offer constructive suggestions. “I really like those blue accents, and what if that button was blue as well?” as opposed to “I really like the blue accents, but the button should be blue as well.” The first variant of the feedback is a more positive, friendly and constructive way of saying this, while staying objective. In addition, it opens up the conversation for follow-up questions and professional discussion, which has more worth than just feedback by itself.
I think plussing is a good way to avoid mixed messages, making sure you come across as more sincere. A mixed message would be something like “This is great work, but …”. The “but” signals someone to disregard the first part of the sentence. Plussing also allows you not to have to label feedback as positive or negative. It's a sincere and objective way of giving feedback.
If you can't plus something right away, when your feedback is going to be negative no matter what, try to express concern instead of something like frustration, anger or disappointment. “I'm concerned this color scheme doesn't work with the product images. We should re-evaluate our earlier choices.” It will help you create awareness for a problem, by focusing on the message.
In a design process, it's important to stay focused on its goals, so always try to frame the direction of your feedback towards use-cases, users, your definition of done and of course your stakeholders. Ultimately, the users of whatever you are designing are what matters, so always try to keep them front and center. You personally don't have to like the colors or layout, interaction or user flow, as long as they do.