The Spoonflower Handbook is here!
The 'Royal Blue' versions in this sample are: two sourced from Wikipedia - Bright Royal blue (hex code #4169E1),and Traditional Royal blue (#002366); and two sourced from ISCC-NBS (stamps) Royal blue 179 (#00416A), and Royal blue 183 (#00304E). Printing these on your preferred fabric will enable confidence in the printed Royal Blue.
Line art, basic repeat.
[Fabric: 150 ppi]