There are many differences of opinion in the world of stranded color knitting - how to hold the yarns, what to do about floats, and what the term Fair Isle even means. There is one debated issue, however, that is experienced differently by each knitter - yarn dominance.
Here are four general yarn dominance "rules". These rules are usually the case but not always.
1. If you carry two colors per row, one color will appear more prominent or dominant.
2. If you are a two-handed color knitter, the yarn held in your LEFT hand will be dominant.
3. No matter how you hold the two yarns (even if you hold two yarns in one hand) the color that strands BELOW the other color will be more prominent. (Look at the floats on the reverse of your work and you'll see that one color's floats will strand above or below the other.)
4. You should never switch how you are holding the yarns in the middle of a chart or motif.
Look closely at something stranded you've knit and try to determine if the stitches with each yarn are exactly the same size. Often when you knit with two colors, one color will be carried more loosely and thus the stitches will be larger. I've even heard a theory that the stitches made with one color look a bit larger because the float above is pushing the stitch out a bit. However if you knit with two colors for a really long time, you'll probably notice both colors even out over time. Knitters who are new to stranded color knitting will probably see the most significant difference in the size of their stitches with each color.
I knit the above swatches to show yarn dominance but I wasn't able to create as much of a visible difference as I'd hoped. Anyway, in the top swatch I held the black yarn in my LEFT hand (where I usually put foreground yarns) and switched it for the bottom swatch. You can see that the black stitches in the top swatch appear a bit larger and the white stitches appear larger in the bottom swatch. HERE is another source that shows front and reverse photos of yarn dominance.
I think the first time I read that the yarn held in the left hand is NOT always more prominent for every single knitter was in Joyce Williams' book Latvian Dreams. I've heard from dozens of knitters on Ravelry who agree that these rules do not apply to them. Some insist they have no dominant yarn at all and some have told me that after a good blocking they could no longer tell which yarn was dominant.
Meg Swansen, Amy Detjen, and Joyce Williams state in the Fair Isle book Sweaters from Camp,"Which color in which hand? For the sake of sanity, we proclaim there is no right or wrong method, since results and desired appearance vary so greatly. The strand carried in the right hand may be prominent for one knitter; another may find the left-hand wool to be dominant and a third knitter may have perfect balance between hands."
How do you determine what colors in a chart to make dominant? That is totally up to you and if you are one of the knitters who experiences yarn dominance you can use it to your advantage to make part of a design stand out more. If you're doing a design with no particular design focal point (like a checkerboard design) or a project with a zillion color changes, make it easier on yourself by choosing ahead of time to just make the light or darker colors dominant.
I'd really love to hear other knitter's comments on this issue, especially from those of you who experience things differently than the usual "rules". What are your experiences with yarn dominance?