Certainly! Here’s a revised color palette with softer tones while maintaining a casual, winter theme that works well with the color #78909c :

 <Soft Winter Casual Dropdown>
1. Done - 
                            
                            #a5d6a7
                         - A soft mint green that conveys a sense of calm completion for the "Done" status.
2. In Progress - 
                            
                            #ffe0b2
                         - A gentle peach that suggests ongoing activity in a friendly manner for the "In Progress" status.
3. Unresolved - 
                            
                            #ef9a9a
                         - A light coral that indicates a need for attention without being too harsh, suitable for "Unresolved" status.
4. Not Started - 
                            
                            #bbdefb
                         - A pale blue that evokes tranquility and potential, fitting for the "Not Started" status.

This palette features softer colors that maintain readability against black text while embodying a wintery, casual feel.

Would you like to explore more options or any other design elements?