I usually find that by the time you're being abducted by aliens you've gone quite long way over the stage of seeing "vivid colours". If you're seeing "beings of light" and "communicating with the dead" on DMT then I think you've got to have seen vivid colours.
Knowing a thing or two about pharmacology, I actually think this is supporting instead of disproving.
If you vaporize and inhale DMT, you have a large peak concentration of DMT, which rapidly declines.
So in the peak of your DMT experience, you only see colours, geometric figures and nothing you can really make sense of.
But when the concentration of DMT decreases, you are starting to see the elves and communicate with the energy around you etc.
Now back to the endogenous release of DMT.
You start off at zilch, and slowly there is more DMT being produced.
This would make it logical that you would start with the less intense effects, that slowly develop towards a peak experience after inhaling DMT.
Although you'd probably never reach the same peak level, since you'd either be dead, or be brought back to the real world.
Perhaps the tunnel effect and out-of-body experience is caused by a different chemical release, for instance an endogenous NMDA-antagonist.
If this takes place
before increase of DMT - which could be likely, since DMT has to be produced from tryptophan, while NMDA-antagonist are omnipresent in the brain - it makes sense to me that people report the tunnel experience when they only have a 'short' NDE, but if they are 'on the edge' for a bit longer, they are getting more of the DMT-like effects.
Again, it's all just speculation.
But I believe this is a lot more interesting and constructive than dismissing everything based on the notion of 'a dark tunnel is not seen during DMT'.
Just as I highlighted before: endorphins are not causing a heroin-high either, but they
do exist.