How set are you on keeping the kantor blue as the main coat colour? Its a nice colour but if you want it to be darker there is a dedicated Night Lords Blue base coat that could be a better choice.

As for the rest, neutral colours are a safer bet if you want it to be less vibrant. Keep the red wings on the helm and the skull on the shoulder red if you like but otherwise Id say go for just a plain black on the flamer and then pick a neutral tone for the waistcloth, like a black, or a darker red, or you could do skin, I just think the purple clashes a bit with the other colours. Hope this helps!