Where and when did he say this?
We can speculate all we want, but in the end, I think the initial reason for the different colours of all Guyvers (0, 1, 2, 2F, 3) was to have a bit of variety and so that you could tell them apart (like most other series which have multiple characters in some kind of suit/armour).
As for why the Gigantic is yellow and not blue... Well, that's when Takaya might have come up with his 'second' reason - host personality.