I have your colouring and I find I need contrast as well as deep and cool colours. So, switching the coat for a mid-tone, even one of "my" neutrals like grey, just wouldn't work on me.
If I did this outfit, I would pick a dark/moody floral on a black background (maybe even all-neutrals, like shades of grey and white on black) and a ruby coat; or maybe cobalt; and black boots.
The alternative would be a black coat, in which case there needs to be lighter contrast somewhere. Either lighter boots (grey? taupe?) or else much lighter colours in the dress (again, pale grey flowers on black). Somehow black in both the boots and the coat seems to be moving farther from the inspiration picture, but if I kept going in that direction, I would end up wearing a black dress and black boots (with maybe a white or grey scarf?) under a bold black and white patterned coat.
For your guidelines on how to adapt, look at Angie's suggestions when she does an ensemble (like today's, or the tomato/burgundy/toffee one); she will switch out which piece is the print or colour, give options for people with different style sensibilities, etc.
