That's true (here's an interesting video on the subject of colored light: https://www.youtube.com/watch?v=uYbdx4I7STg), but as mentioned at https://youtu.be/AF8d72mA41M?si=i8wjHjaKRaQbf23b&t=1516, once you have a blue LED, you can use a phosphor to convert the light to a range of longer wavelengths.