You can color polyester resin by adding pigments.
Pigments can disturb the mechanical properties of resins.
Use as little as needed, maximum 1%, stir thoroughly.
Saturation and opaqueness depend on the amount of pigment used.
Make small samples to get an idea.
The coloration of the pigment paste can be different to
the mix of resin and pigment. See the blue pigment paste
in the picture appearing more purple like, or the turquoise
looking fairly dark / blackish.
Top picture shows the different colors used on a 4oz piece
of fiberglass on a PU foam off cut (the green being laminated over the stringer).
Middle shows blobs of resin mixed with ca 1% pigment paste.