If you are someone who lives in one colour and that colour is black, I have a few ideas on how to add colour without feeling like a walking rainbow.

Start small and add one pop of colour although it can be muted and not bold. This can be done by adding an accessory - ideas can include belt, necklace, earrings, ring or a brooch.
A scarf is a simple way to add colour and also can add texture - first check to see how that colour looks up against your face, you want to be careful not to drain colour from your face by wearing a hue that does not complement.
As we age black can look quite harsh against our face, draining colour instead of providing the glow we may want. An easy fix is to move to a navy or brown colour tone.
Add your pop of colour in the form of a top or shirt where only the collar or sleeves are peeking through, allowing for a glimmer of colour to complement the rest of your look.
Make-up is another way to bring to life an outfit and what immediately comes to mind is a bright lipstick particularly red, pink or orange that adds a statement without having to change up your wardrobe.

Black is a safe and easy go to colour and it make sense to be a core staple in the wardrobe, although colour will create that pop that can bring to life an outfit.