I was noticing that, right now, the Princess is always in white/pink colors, but in the future she will be wearing black, with we see in a variety of characters, I think it means the lost of innocent/purity, etc.

But going back, yes, it's true in the raws there is some friction between Dowun and the Princess, but I don't think he is going to backstab her, at least not easily. Not because I think he likes her or they lived together for long, but because of the letter.
The reason why she gets authority to overthrow her brother is because of the letter, probably making her the new Empress. Dowun is a man and her husband, but he is no royal blood, if he was crown like the Princess was going n the scene, getting rid of her wouldn't be much a problem (I don't really know how is the story of China, accesscion in power, royal family, etc. It's just my interpretation of the raws). But, what we see is that she has the power and he is by her side, which means he can't just do whatever he wants, unless he have the whole council in his pocket, that is another history them (I believe, again).

from what I can tell they do get the thronne but theres some friction between dowun and her. And add on the fact that they did teccnically overthrow the current ruler i wouldnt be surprised if he backstabs her in the end to become the true emperor. He did throw his brother under the bus so I wouldn't put it past him. Or the other way is her killing him after she realizes his ambitions are too strong