Ramen noodles are traditionally coloured by kansui
https://www.finecooking.com/recipe/homemade-ramen-noodles
although some manufacturers might replace kansui with egg to produce much the same result.
Theoretically you need to place packet ramen noodles into boiled water and leave them for several minutes to absorb the liquid. I suspect that you could get much the same result though if you put them directly into a pre-prepared soup, as long as
(a) the soup was hot enough ; and
(b) you left them there for long enough.
The only problem I could foresee might be that you'd need to keep the soup at a temperature very close to its boiling point but without actually letting it boil (which would impair its flavour).
Given that several of the major supermarkets sell Naked Noodle soups at prices between 30p and 50p though, it might be simplest just to buy such a product. e.g.
https://www.supermarketwatch.co.uk/product/naked-noodle-ramen-noodles-thai-chilli-chicken-soup/LSW461