Hi there! It sounds like the cells you're having trouble with might have conditional formatting applied to them. Conditional formatting can override manual formatting changes, including font color. Here's how you can check and remove any conditional formatting:

1. Select the cells you're having trouble with.
2. Go to the "Home" tab in the Excel ribbon.
3. Click on the "Conditional Formatting" dropdown menu.
4. Select "Clear Rules" and then "Clear Rules from Selected Cells".
5. Try changing the font color again and see if it sticks.

If that doesn't work, there might be other formatting applied to the cells that's preventing the font color change. Here are a few other things you can try:

1. Check if the cells are locked. If they are, you'll need to unlock them before you can make any changes. To do this, right-click on the cells and select "Format Cells". Go to the "Protection" tab and uncheck the "Locked" checkbox.
2. Check if the cells are part of a table. If they are, you might need to change the font color for the entire table instead of just the individual cells. To do this, select the table and go to the "Table Tools" tab in the Excel ribbon. Click on "Design" and then choose a new font color from the "Table Styles" dropdown menu.
3. Try copying and pasting the contents of the cells into a new worksheet or workbook. Sometimes formatting issues can be caused by corruption in the file itself.

I hope one of these solutions works for you!