Colour has always played a part in Apple's product strategy with certain colours being sold as exclusive or limited editions at a premium, as well as new colours, preferably red, to inject more interest into older or plateauing product lines.
For 2018, some analysts believe Apple will focus its more affordable iPhone line into more colourful blue, yellow and pink models. This will be for the possible replacement of the iPhone SE, which is the current more affordable basic iPhone which is almost two years old. StreetInsider speculates the coloured iPhones will resemble the iPhone X, but will have more affordable LCD displays,
Will Apple revisit colour like it did with the iPhone 5c? It seems an odd step back for Apple which always looks forward but if this is a strategy to make more affordable iPhones available without cannibalizing the more expensive flagship lines. I would take this rumour one with a grain of salt.