The one tip that helped me the most before I started taking color theory classes was to google search for my main color plus the word "flowers". For example "yellow flowers". Then, pick my remaining colors from combinations that occur in nature, plus any neutrals I wanted. Yellow is usually considered a neutral anyways, so you may want to choose another color to search for and go from there, but here's the basic search I did:
https://duckduckgo.com/?q=yellow+flo...ax=1&ia=images
As you can see, depending on which flower you go with, you can end up with oranges or blues or greens or browns, reds, pinks and so forth. Here's one example of what I might pick:
https://images.duckduckgo.com/iu/?u=...%2B(2).jpg&f=1
Colors: yellow, gold, yellow-orange, red, red-orange, green, yellow-green, white, black (could also add browns/tans)
In my experience most parents don't have problem with adding a little bit of blue to a baby quilt for a little girl -- especially since it has a flower on it. So you could always add a bit of blue. I'm not really sure you need a whole bunch of colors, though. You could add interest by picking different shades/tones/tints of other colors (yellow has the least variation in it of all the colors) or by changing the scale and/or visual texture of the prints. For example, in the link you provided, they have some large scale floral prints, those relatively small dots in the center of the flower, that pink on pink flower/vine(?) fabric that's 5 up from the bottom left corner, the close-together stripes that repeat in different parts of the quilt, and the bright pink, yellow & orange solids that really draw your eye straight to the flower.
If you attach a photo of the fabrics you've already picked, we could probably provide more suggestions about how to add interest to the quilt while still maintaining unity.