Good neutral background for testing materials

[FutureMan]

I'm trying to tweak my glass and water materials, however I need a background\plane for it to cast caustics onto.

In this case I'm just using an oversized box with flipped normals and a solid color, however it's kind of hard to tell how good the materials actually look. In the end if I use black, grey, or white it is hard to tell with the clear materials. Other colors make it even worse and can affect how colored glass/water looks.

So what's the best background to test against? A solid color, if so what color? Maybe some kind of image? What image?

[Torlok2002]

with glass, try a checkerboard material. You can see how it refracts the image.

[FutureMan]

It kind of works but not really. Actually the cellular gives a much better effect if you turn the size and spread up.

I guess I'll just have to try a variety and see how it looks with each one.