With existing Flash Purple customers about 6 out of 7 have phones that will only work on Flash Purple.  The model numbers noted of the following universal devices will work on Flash Yellow and Flash Green:

Apple: iPhone X (A1865), iPhone 8Plus (A1864), iPhone 8 (A1863), iPhone 7 (A1661), iPhone 6s Plus (A1687, A1634), iPhone 6 Plus (A1524), iPhone 6 (A1586), iPhone SE (A1723),

 Moto E4 Plus (XT1775), Moto E4 (XT1767), Moto G5 Plus (XT1687), Moto G4 Plus (XT1644), Moto G4 (XT1625), Moto G4 Play (XT1607), Moto X Pure,

 Google Pixel 2 (US version), Pixel 2 XL, Pixel, Pixel XL, Nexus 6P, Nexus 6, Nexus 5x,

 Samsung S7 SE (SM-G930U), Galaxy S7 Edge SE (SM-G935U), Galaxy S8 SE (SM-G950U), Galaxy S8 Plus (SM-G955U), Galaxy Note 8 (SM-N950U)