1. Change all font into the same color
If you only need to change the font color of Y-Axis labels into the same color, please select the Axis, and change the font color in the Home Tab from the Ribbon. Please see how to change the font of Axis Labels.
2. Change Positive to black and negative to Red
If you want to change the Font Color to different color: for example, black for positive numbers and red for negative numbers.
Step 1: Right-click the Y-Axis, and select "Format Axis" from the list;
Step 2: In the "Format Axis" window, select "Number" in the category, and select one of the red color for "Negative numbers".
3. Change the font color based on value
You can also change the font color based on value: For example, change the Font color to Red when greater than 15; and green when greater than 0; and leave others as default.
Step 1: Right click Y-Axis and select "Format Axis";
Step 2: In the "Format Axis" window, select "Custom" in the category, and paste the code "[Red][>15]#,##0;[Green][>=0]#,##0;#,##0" in "format code" box.
Step 3: Click "Add" and the font color in the chart now is changed.
