coolGray property Null safety

1. nativebase_flutter
2. color_model
3. NativeBaseColor
4. coolGray property

NativeBaseColor class

1. Constructors
2. NativeBaseColor
3. Properties
4. amber
5. black
6. blue
7. blueGray
8. coolGray
9. cyan
10. dark
11. darkBlue
12. emerald
13. fuchsia
14. gray
15. green
16. hashCode
17. indigo
18. lightBlue
19. lime
20. orange
21. pink
22. purple
23. red
24. rose
25. runtimeType
26. teal
27. text
28. trueGray
29. warmGray
30. white
31. yellow
32. Methods
33. noSuchMethod
34. toJson
35. toString
36. Operators
37. operator ==