Flutter如何为下拉标签和下拉列表文本设置不同的颜色？

I am trying to use dropdown, and want to have Colors.white for the selected text, and Colors.black54 for the text of drop down list. But when I tried to use color attribute and change it to White, it also change the color of drop down text.

DropdownButton<String>(
                    //this changed the color of both text, intial text as well dropdown text color
                    dropdownColor: Colors.white,
                    value: value,
                    style: TextStyle(color: Colors.white),
                    icon: CircleAvatar(
                      radius: 12,
                      backgroundColor: Colors.white,
                      child: Icon(Icons.arrow_drop_down),
                    ),
                    items: places.map((String value) {
                      return new DropdownMenuItem<String>(
                        value: value,
                        child: new Text(
                          value,
                         //I tried giving text style here , but same result 
                          style: TextStyle(color: Colors.white),
                        ),
                      );
                    }).toList(),
                    onChanged: (_) {
                      setState(() {
                        value = _;
                      });
                    },
                  )

这是它的图片。