Growing hybrid flowers can be a tough journey, but with this guide we'll explain how flowers work and the colors you can get from crossbreeding and pollinating with cosmoses, hyacinths, lilies, mums, pansies, roses, tulips, and windflowers to create some unique blue, pink, and even black flowers. With any luck you'll even get the golden roses! Use the below chart to view the chances & odds for growing the color you're aiming for. (Think of it similar to a multiplication table!) If you're looking to get a bit more of an in-depth idea of how the flower genetics work, you can also check out this fanmade guide.
Information taken from various sources, including the Official Animal Crossing: New Horizons guidebook. Thanks to all who have helped!