60% of my items have forced first sub attribute as the main color as the core one, a bit less than 10% are rainbow, the rest is everyting in the same color or just two (either first attirbute is different color but the second one is the same as core, or both are different colors so not rainbow). So yes the actual rainbow items of everything different is rare now.
You can take a look at some results, with counts and screenshots here: https://forums.ubisoft.com/showthrea...5-Rainbow-Loot
TL;DR - for me, around 60% has a first attribute that matches the color of the core attribute. About 15% seem to be full rainbow.
As always, your RNG will vary.