Solved it.
For the benefit of others, what we were doing wrong was trying to use Control4’s built in lighting scenes to change colours on the Hue bulbs. It worked but the transition was usually abrupt, no matter how you configured the ramp numbers.
The solution is to use the built-in presets feature of the Hue colour lamp group driver (available as actions in the Programming tab). You first set the lamp to the colour you want by manually dialling in the red, green and blue channels and then save it as a numbered preset by going to the driver’s Actions tab (in System Design), and then you recall that preset through programming whenever needed. There is a ramp setting there, which works perfectly.
Furthermore, if you want to set up a party mode where it randomly cycles through different colours, the driver already has an option specifically for that, so you don’t have to mess about with presets either.
All in all, the whole thing works extremely smoothly, and I would be happy to recommend Philips Hue bulbs and downlighters to my clients in lieu of using standard fixtures hooked up to Control4 dimmers, particularly if they want colour or temperature control. Of course, it would be more expensive to go that route in a lot of situations, so it is not always the best choice.