Corner triangle color suggestions needed
 

I am working to finish up a UFO from I think, 2013. Anyway, mystery train quilt #6 from quite some time ago. I am stumped as to what color to use for the side triangles. It is such a mixture of colors that I am not sure what to pick up in the triangles. Also if you see some glaring need to move a block or two around, please mention it. I just stuck the blocks up on the design wall. Thank you and all suggestions are welcome. Love this board.
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I would start pulling neutral colors (golds or tans) out and audition each one to see what looks the most pleasing to you.

I actually like the white of the design wall. Makes the blocks seem to float.

Even black to frame what you've already done would look nice!

I would use a dark deep purple.

I'm going to make a different suggestion. I would make more blocks in a size that would work and cut them to fit the spaces. Or at the very least make the triangles with a colored square in the point, in which case I'd make the other part of the triangles in white. I think too much of any color as those triangles will detract from the charm of the quilt.

I was thinking that also as I was looking at the picture. I would go with a triangle using that design to fit in each space then a border.

Please show us the end result.

I would make the triangle and the border from the same fabric. It will float the design and be stunning.

I like your jumble of colors, they look just right to my eye. I notice that you have a very generous amount of neutrals in tan, grey, brown, cream. I have two questions for you: What fabric will you use for the backing? I think a neutral would be very nice. And the narrow white strips that separate the colors, Will you also put a white strip around all your blocks, so that the outline continues to the edge of the Triangles? I hope the answer is yes. Then, I would choose 5 colored fabrics for your side triangles, and mix them up all around your quilt. I imagine this with a tan/brown backing and 5 various neutral colors making up the Triangles--that's what I would do. OR--if this is a gift ?? Ask the recipient what color they would choose, let them be your co-creator. That's half the fun.