Allow choosing of "No Color" for color picker

### Feature Proposition
Allow choosing of no color to remove courses that one may not care about. This complements the incoming custom course feature in #3373 

<img width="619" alt="Image" src="https://github.com/user-attachments/assets/73db73fa-8c4d-4241-abea-a460720767d5" />


Can be done using `transparent` parameter in `TimetableCell.tsx`