Unwanted colours in stretched transparent graduations.

I have a long 10m prim and I'm applying one quarter of a texture onto the prim.

This texture goes from a small amount of white (in the transparancy) at the start, to total transparent at the end.

However, when you view this (esp with midnight lighting on) you can see faint coloured patterns in the middle where the white is gradually fading out to fully clear.

not a lot. Sort of like the faint colours you get in a soap bubble.

Definate bands, but all blurred at the joins between the very faint colour changes.

Is there anyway to avoid (or minimise this effect)?

I assume (due to pallet restrictions) there are just not that many shades of transparent white to cover the area I'm stretching the texture over, so it's creating these colour errors thru some dithering/blurring process.

Can you post a snapshot? That's a very odd effect, and it's hard to tell what you are seeing from the description you gave. For the project you have been describing, your texture, in Photoshop, should consist of one layer (filled with white) and an alpha channel (graded 50% gray to black). There are no colors involved.

Well, you've got 256 possible transparency values. That's more than enough for a smooth grade.

As Rolig said, it's hard to tell from your description (a screenshot would definitely be helpful), but my guess is you're seeing the effects of in-world lighting conditions. What happens if you set the texture to full bright? If the colors go away, then you know there's nothing wrong with the texture itself.