I have heard of the 6 month rule as well.
When you buy a car normally in the states if you had it registered somewhere else (another state) prior to registering it in the new state you were excluded from sales taxes in the new state. I know some people go out of state to buy then bring it back where it is finally registered in their name and then owe taxes.
I technically have the car registered and have put miles on it so it is no longer used. I see no difference. I understand paying ownership taxes, but paying sales tax is another story. Even more so since sales tax will be $3,000 - $4000.
My resident state is Montana which has no sales tax, but requires the vehicle be brought to my home county to be inspected. This is really fine, but a last resort if I can't get around the Colorado problem. Colorado also requires residents including non-residents to have Colorado plates.