I love black tapware and I think it will be around for a few years. It seems that black, copper and all things coloured are making big waves. Black is such a great complementary bathroom colour for all the brights that are also trending and probably here to stay for a while. https://www.google.com.au/search?q=methven+coloured+taps&espv=2&biw=911&bih=445&source=lnms&tbm=isch&sa=X&ved=0CAcQ_AUoAmoVChMItvbI8qvlyAIViZqUCh3kvg1z#tbm=isch&q=coloured+tapware&imgrc=26qdG_I1UN7lWM%3A

Personally I would always go for a consistent style throughout the house if at all possible. Expensive if you have to make a lot of changes to achieve it. I guess it comes down to how much it bothers you.

Yes just look for a new bath spout and a new shower head set up. Take a photo to your plumbing supply company and they will recommend a good retro fit design. I would not replace the mixer in shower and bath. Just clean it with gumption. Unless you want to go a completely different profile or colour. Then yes replace as is, otherwise it will be a costly plumber job.

In my opinion, although black tapware is not as trendy as brushed nickel or gunmetal, it is timeless that will definitely match with a MCM style bathroom, plus it also matches with your door handle. Gold is also good as it can add a warm tone to your bathroom that will make it feel more uplifting. Both colours are fine, it you can't decide, you might choose the one that will match with the colour of your vanity. Just keep in mind to choose a matte finish rather than a gloss finish. We have tapware in all classic or trendy finishes, check out if you want: https://fontaineind.com.au/tapware/rosa You can also get some colour samples from us for comparison: https://fontaineind.com.au/rosa-colour-sample