redux: Component won't re-render (universal)

I have an action posting to my database, with the reducer successfully posting the response to my state. This works fine, however my component isn’t re-rendering automatically, I can only use the updated component with a manual re-render.

Here’s my component:

export default class Colours extends React.Component {

  componentDidMount() {
    this.props.getColours();
  }

  render() {...}

  addItem = () => {
    this.props.postColour({ 
      colour: 'pink' 
    });
  }

}

my action:

import req from 'axios';

export function postColour(value) {
  return {
    type: 'POST_COLOUR',
    promise: req.post('/api/colours', value)
  };
}

and my reducer:

import Immutable from 'immutable';

export default function colourReducer(state = new Immutable.Map(), action) {
  switch(action.type) {

    case 'POST_COLOUR': {
      return state.merge(action.res.data.colours)
    }

    ...
  }
}

Am I missing/doing something completely wrong for my props not to be updated as soon as the response is merged with my initial state?

Thanks!