Whenever I have a project go off the rails, I always take extra care to perform a post mortem. I know the natural inclination is to just cram the thing in the back of your closet and pretend like it never happened. But before you do that, it can be very helpful to spend some time giving honest consideration to what went wrong and - most importantly - how to avoid that problem in the future.
Rules of thumb to help you decide.
In the case of my latest project, it involved stripes, and I decided to choose the colors as I went along. And it did not end well.
Now there is this theory floating around that you can just randomly choose colors out of a basket and it will all work out great. In my experience, that is a quick recipe for a disaster. As near as I can tell, that method only works when you spend a lot of time choosing which colors to put into the basket in the first place.
A smarter person would have swatched the colors first. Mea culpa. I couldn't bring myself to spend several hours swatching, and it basically cost me the entire project. At one point I ripped back an inch of knitting to swap out a color, which represented about six hours of knitting. I would have basically reaped the reward twofold if I had swatched first.
I recommend swatching your color combos first. Colors do weird things when you put them together. I promise you, the next time I am faced with a striped project, I will be swatching those stripes. Learn from my trauma.
One lesson I have had to learn repeatedly over the years is that it works well to vary either HUE or VALUE but NOT BOTH. "Hue" refers to the color, and "value" is how dark it is. So you can collect a lot of different colors, as long as they are all light or all dark (like light blue, light green, and light orange). Or you can choose one color and use different shades of it (like light blue, medium blue, and dark blue). But you want to be very cautious when combining, say, light blue and dark green.
Color "families" are often grouped along these lines. When you think of pastels, jewel tones, and neutrals, these are collections that are grouped by either hue or value (but not both).
And finally, take into account the proportion of the colors. Will it be one main color, and then smaller amounts of a contrasting color? Or will it be evenly split between the two colors?