Black will hide the best and be the most subtle. But you'll have a time working with black on black

For subtle I would use black.

I'd use Superior Threads' "So Fine" Gunmetal Black, which is a nice greyish black, thin, and wouldn't interrupt the white areas. Once quilted, you won't even notice.

gray

I quilted a similar one in variegated blk/whte it came out fab. Whatever colour you choose the quilt will look awesome.

Gray or varigated black/white

I like the gray idea.

Ranger, the pattern is a disappearing 9 patch. Wanted something fast as had just done a jewel box queen for my sister.
Thanks for your lovely comments and suggestions. Went to Hobbycraft this morning. Got black thread and also found a variegated black/white/grey. Got a sample sandwich made so will see which looks the best.
As previously mentioned we do not have a large selection of shops near here, and as I do not drive I rely on my DH to take me to the stores. Bless him he is very supportive of this expensive hobby. Mark you he fishes and plays golf so I guess it is all a matter of balance.
Sue

I have a black/white/gray varigaited thread that would look cool on that quilt.

Red. It's a dramatic quilt. Needs some drama in the quilting.