what color thread?
 

So the majority of the quilt is dark colors but the stars are beige, the back is red. What color thread to machine quilt? Thanks

You have options:
1. Use a variegated in the top and red in the bobbin. Some of the variegated threads have really short transitions which means you would have an even distribution of visible versus blended appearance.

2. Use a a monopoly in the top and red in the bobbin. Adjust the tension so the red won't pop to the top.

3. Use a neutral in the top which would show on all parts (more so on the dark) and a red in the bottom.

4. Select threads to match the color in each section and change the thread accordingly. I'd still use red in the bobbin.

Have fun.

Wish you had posted a photo. Quilting on my Bernina, I do change my top thread to match the fabric.

I agree, a photo would really help, but based on the information you've given ...

if you don't want your quilting to show a lot, I'd probably go with a grey in a 50wt
if you want your quilting to barely show, go with a muted variegated thread that is not an exact match for any of your fabrics (or else it can look like you forgot to quilt part that part)
if you really want to show off your quilting, go with a 28 or 40wt and a beige everywhere but only outline the stars. Use a red inside the stars (it won't show on the back, but you will still see the stars' outline & that should look nice).

I would recommend a med. gray on both top and bobbin. Do you have a scrap sandwich to try out different colors? If not fold the quilt so you can see both the front and back, then pull thread off the spools, pooling it around on the top and back to see how it plays.

I'd go for the beige. Just quilted one with a minor color and it really sparkles.