Improved flow on Picker.Item

Summary:
Flow improvement: Added ColorPropType on color property on Picker component
Closes https://github.com/facebook/react-native/pull/12528

Differential Revision: D4619897

Pulled By: ericvicenti

fbshipit-source-id: 6356d6f4ae6d2db3d93a56882ec51d140f9ac2ab

Libraries/Components/Picker/Picker.js

@@ -134,7 +134,7 @@ Picker.Item = class extends React.Component {
  props: {
   label: string,
   value?: any,
-  color?: $FlowFixMe,
+  color?: ColorPropType,
   testID?: string,
  };