1/4 done, but now I'm concerned it's going to be too dark/not have enough contrast. What do you think. It's navy, green and muslin.

I can definitely see the colour difference and it is still giving the 3-D effect ... just a little more subtle than some. It works.
You might want to consider what lighting it will be in ... will it be in a dark area/room and the colours become more blended? or in a very bright area, where the colour difference will be easily seen?

I like it ... though it depends on what effect YOU want to achieve!

It's always hard to tell in photo's, they tend to be dark anyway, how does it look to you in a bright room?

Here's an additional picture. I think it'll be fine

you can always enhance the contrast when you quilt by using lighter thread in similar colors-looks good so far

I think the contrast is low but I agree with Alfosa that you can probably highlight one of them with a light coloured thread. It looks good.

I like it very much and think it will be beautiful as a quilt. Looking good so far.

Contrast is low but enuf that it will be lovely

I really like the darks against the light background. Keep going. Looks Great.