it just takes time-practice- suggestions- advice- and sometimes the help of a color wheel.
all that being said-
look at nature-
in nature colors you would (think) could never be together go together wonderfully- look at the sky- if you tried to paint it = it would look- fake- but if is not fake it is beautifully painted!
i used to always have to ask the guys in the family to pull fabrics for me for a new quilt- they seemed so good at it= then dog-gone it they grew up and moved away- leaving me to have to figure it out all by myself-
one thing that can help is to do what other (artists) do=
disregard color- use a gray scale- and design your quilt using VALUES- without worry about color-
as in...separate your fabrics according to light-medium-dark---sometimes i will lay out my fabrics with a couple inches of each showing and take a picture- then print it in grayscale- if a fabric does not (work) it becomes quite evident.
and all colors/prints can go into a quilt together as long as the values are balanced. with time you will find it easier and easier to choose fabrics.