Correct. The strain may have a tendency to do so, but unless it is a trait that has been "fixed" by genetic selection, not all plants will express the colour change. Anyway, colour change does not have any affect on potency or yields and is just a visual piece of "eye candy". You could try and lower the temps even further to try and induce the plant to do so as well, but as they are close to harvest, time is probably against you. Plants that change colour at maturity usually have a chance of doing so of somewhere in the range of between 60% and 80%, and, unless as stated earlier, the trait has been "fixed" through genetic selection and stabilised through many generations of breeding, getting a plant to do it without being genetically "fixed" can be a bit "hit or miss".
Hope this helps,.... Organoman.