Are you like me, that you have a specific spectrum of colours that are your "Happy Place" and that you tend to re-use throughout a lot of your projects? My colours are definitely the greens and blues and all shades in between.
Even within our own comfort colours though, we need to make sure that anytime we work on a colourwork project, there is enough contrast between the colours to make sure that they pop and that the hours you invest in your project will show it off in the best possible way.
I wouldn't call myself an expert in colour theory, I am definitely still learning. There is a big book lying on my to-be-read pile that goes into it in a lot of detail, and maybe once I have worked through it, I might write another post about the colour wheel and all that good stuff.
For today though I am sticking to a few tricks that I use, to make sure that there is enough contrast between the colours I want to work with.
When looking for yarn for your colourwork projects, as opposed to a fade or a project with bigger blocks of colour, you want to make sure that there are not too many overlaps in colours between the two skeins. I tend to also pick at least one semi-solid skein, as otherwise the different stitches will not stand out.
First, I open up the skeins that I am looking at working with, unless I am working with a commercially dyed yarn. This is to make sure that there aren't pops of colour hidden inside one of the twists. You will be amazed how big the difference can be, between the wound up skein of yarn and the opened up version.
This alone already gives you a great visual clue into whether the yarns really go together.
I take pictures of the different colour combinations I'm looking at, and then I edit the picture to black and white on my picture editing app (I use Snapseed, but there are many others out there). You can also use other setting to play around with, like the Noir tool, which gives you different versions of greyscale, but usually the black and white is my go-to.
Here are a few examples, the first two would make great colour combinations for a colour block project (and keep your eyes peeled, because that is totally what I'll make with them), but the last combination is what I would pick for a stranded colourwork project, like Mosaic or Fair Isle.