I hand quilt and use many different color threads in a single quilt quite often. Doing it now as a matter of fact. I agree with the others to not do SITD. If you're taking the time to hand quilt, then let those stitches show! You'll have consistent stitches in no time. If you're wanting to do 'waves' in the water - which I think is a great idea, btw - try looking in the scrapbooking section of JoAnns or other craft stores for a wavy edge ruler. Makes that process easier and more consistent (if that's what you're looking for). Or find shapes or whatever from coloring books, etc. and just make yourself a cardboard template of the outline for your quilting design.

I am making waves in water, have grain lines for boat and a rope border. Any suggestions for the sail and sky
area?

First of all, you did a great job!! I admire the fact that you want to hand quilt it. I would put details, like waves and clouds in the corresponding colors.

How about slightly curved line(s) in the sails? Looks like the wind is taken them. Maybe wavy kind of clouds in the sky.
Have fun, it looks beautiful!

Great job and I go with the off white.

You did a great job on your first try at Triangles. A very cute quilt.

If you are going to stitch in the ditch it won't matter about the contrast in colors. If you don't like a white or neutral thread try a little stitching in a medium grey. Gray has a great effect of just disappearing on most colors.

Show the completed project, please!
peace

EDIT: as for quilting ideas if you have access to any clip art you can search for specific things like wind or rope or anything you want to put thread to, and get some cool ideas that you can transfer into your design

I did that very thing for the spider I quilted on a spiderweb quilt. I get loots of ideas from simple clip art.

Very pretty!

I would use different colors to match the fabrics. Stitch 1/4" from seam in triangles and as you please elsewhere.