I have to tell you, I have been having the worst WORST luck with knitting projects lately.  And you know why?  Because I ignore that little voice inside my head that's disagreeing with whatever decision I happen to be making at the time.  I hate that little voice, but it always turns out to be right in the end.  (Which only makes it smug!)

People, this kind of thing is why I always knit with gray!  (Cascade 220 shade #8400 "Charcoal.")

They teach you primary colors in school.  Eventually you grow up and move through the world and learn about secondary and tertiary colors (if not that they have official names like "secondary" and "tertiary").  Probably at some point you decide that "complementary" doesn't necessarily mean "goes well together."  Which is true - I would say that "complementary colors" actually fight loudly with each other.  

And then you start knitting, and you're adrift all over again!

I have made more than my fair share of bad color choices with stranded knitting.  Fair Isle designs are typically worked with a very dark color and a very light color, and there's a reason for that!  I knitted a fake Fair Isle hat once in blue and green.  Sounds nice, doesn't it?  Blue and green?  The problem is that in an intricate colorwork design, the blue and the green were virtually indistinguishable.

This is where the term "value" comes in.  Value can best be explained by mentally switching a color to black and white.  If you took a black and white picture of the blue and green yarn skeins together, you wouldn't be able to tell which is which.  Their color value is almost the same.  

When you do colorwork in knitting, you want to be sure to choose yarns with a lot of value contrast - darks and lights.  For a lot of projects, the actual color matters less.

The other thing to consider is that light colors show texture a lot better than dark colors.  This again is related to color value.  It's all about the difference in color value between a color and its shadow.

Imagine knitting a single purl stitch on a swath of white stockinette, and a single purl stitch on a swath of black stockinette.  The bad stitch is going to show up much more clearly on the white project.  A shadow against white stands out a lot more than a shadow against black!

Stitch patterns and textures like cables and some lace projects rely on shadows to make themselves visible.  Knit a black sweater with cables and it will feel neat, but you'll hardly be able to see the cables.  (This is part of the reason why traditional Aran sweaters are worked in a cream or light gray yarn.)  

Tweed and multicolor yarns, as pretty as they are, can have a very confusing effect on the eye.  I once knit a fairly complicated lace scarf in a tweedy yarn with lots of flecks of color.  Total waste of my time!  You couldn't see a bit of the pattern.  Sigh!