Coordinating yarns in my stash is not difficult. I am a big fan of blue - if I sort my Ravelry yarn stash by colour more than half of the entries are either entirely or predominantly in shades of blue. My difficulty is in finding colours that are different enough to contrast with one another but that will actually go together.
In the end, I put together all the part-balls with enough yardage in them to be useful, took a photograph and then converted it to black and white. That way, the colour of the yarn is less of a influence and instead you see the colour value. The colour value is not about the hue, about whether it is blue or red or green. It refers to the lightness or darkness of a colour, and for colourwork you need colours with differing values that will stand out against one another in a pattern.
There is also a warmth associated with colours. Warm colours, those based on reds, yellows and oranges, can appear to advance towards you, as can brighter colours. Cool colours, the blue-based and green-based ones, can recede into the background, as can darker colours.
I wanted to choose a lighter, brighter colour for the areas that I wanted as the foreground of the colourwork design and the coolest colour for the background.
So what was this combination of colours?