Hi Iryna,
It depends if you want to be literal or more metaphorical. For metaphorical, I rely on imagery, pick something from nature that everyone knows and compare the color to that.
For example, some Claude Monet paintings have very soft tones, and I might say the blue he uses is soft like the early morning sun, or the yellow is light like a newborn chick's feathers. It can also be something that's not literally a color, but evokes a certain feeling, like "the green of the grass is soft like a refreshing breeze on a hot summer day."
If you want to be literal, the below posters are absolutely correct.