I never use black or white thread unless it is a black or white fabric. Grey is a wonderful color to use but I also use a light lavender or a beige. All of them will effectively blend in for piecing.

I typically use either grey or beige or some variation of those. Works well with just about any color.

For all of my piecing, I use white or cream unless there are two black pieces sewn together, at which point, I switch to black thread. I rarely use colored thread, unless applique-ing (sp??). I also will use colors for quilting.

I use medium grey on everything. It really just seems to fade away.

Are you talking about piecing or quilting? for piecing I use a neutral thread like ivory or off white. Gray might work nice since the colors are bright. For quilting - depends on how you are quilting. There are verigated thread in the colors that you are using. Also I use invisible thread on top for SID. It blends in so well that you can not see any top stitches. on the bottom I use thread matchng the backing. Just be careful if you're using dark thread on the bottom and invisible on top that the dark does not show on top.