As far as I know, the only way to change the color of the 'Date picker' is to change the actual theme of your canvas app. However, changing the theme impacts all object within the app which is not desireable after hours of design customization. Also, the standard theme colors to select among is perhabs not in line with the customized design theme chosen for your app.
Suggestion is to implement a property of the Date Picker to customize the fill color when selecting the actual date. @D365Ideas_Admin