The creators of the Expanse have done an excellent job, not only concerning the story (and the characters, special effects, music, so all of the series were perfect to me), but the costumes as well. I took a look at one of my favourite character's wardrobe change throughout S1-S3 - and analysed according to my perception.
Season 1
black: it is considered a negative color and usually symbolizes death, grief, or evil but also depression.
Season 1 had a different coloring, than the following seasons. It was darker, the colours were mainly grey, blue and black. We meet Errinwright, who seemes a powerful, but a mysterious man, former protégé of Chrisjen Avasarala. His colours were mainly black, and dark grey (except for one scene, where he wore blue). In contrast of Avasarala's colourful saris, his shirts and suits were plain, elegant and dark. Errinwright costumes suggested that he is a man of power, but there is something dark about him. We did not know much about his character, but we DID realise that Shawn Doyle portrays him perfectly.
Season 2
blue: It is the color of the ocean and the sky; it often symbolizes serenity, stability, inspiration, or wisdom.
Season 2 changed the colouring, and the series became brighter, with more vivid colours. This had an effect to the character of Errinwright too. He had more screentime (thankfully) and got more dephts, he became a three dimensional chatacter.
He wore steel blue and blue suits, which made his appearance more striking. He was no longer in the shadows, we even got to see his vulnerable side. Trhoughout season 2, we could not decide, whether he was on a good or a bad side (and frankly, we hoped he might choose the right path). And when he chose to kill, he was in his black suit again..Anyway, I am thankful to the costume department, that they reliased, blue fits Shawn better!
Season 3
Grey: Gray can be perceived as emotionally distant or disconnected, and it’s associated with morally questionable matters as well.
Errinwright chose to betray Avasarala, and start a war between Earth and Mars. Chrisjen, his previous moral compass was not around anymore. He wore more grey and dark suits again, which kinda represented that he was shceming again in the shadows, controlling (and despises) the secretary general. (Interestingly, in the railgun strike scene, he wore blue, but at that scene, he was shocked and angry because of the error the SG made).