Do you have a digital camera? (or any camera, for that matter)
Take photos of various layouts and look at them.
Things sometimes look "different" in a photo than when just looking at it in "real"
Or one sees something differently - also if you can print up (or view) some of the pictures in black and white -
As someone else has already mentioned, it's also a good idea to take a picture of it "the way you want it" so that you can refer to it -
I also like it better with the lighter shades of purple added.
I think the more shades of the colors you are using, the better - because then the project will "go" with more things. (The reason for this statement - I am working on something with greens and lavenders - that I started years ago - and the green I'm using is really just "one" green and, with hindsight, I would have been less "matchy-matchy" with the green)
I agree with you and the others - that one piece is distracting and doesn't "go" with the rest of the fabrics you are using.
As far as the seam allowances go - if you are cutting your pieces all the same size - if you are consistent with whatever seam allowance you use - it will turn out okay.