We can easily change all labels' font color and font size in X axis or Y axis in a chart.

Just click to select the axis you will change all labels' font color and size in the chart, and then type a font size into the Font Size box, click the Font color button and specify a font color from the drop down list in the Font group on the Home tab. See below screen shot:

Then the font color and font size of all labels in the selected axis are changed at once.

Apply conditional formatting to fill columns in a chart

By default, all data point in one data series are filled with same color. Here, with the Color Chart by Value tool of Kutools for Excel, you can easily apply conditional formatting to a chart, and fill data points with different colors based on point values.

If you want to change all negative labels' font color in X axis or Y axis in a chart, you can do it as follows:

1. Right click the axis where you will change all negative labels' font color, and select the Format Axis from the right-clicking menu.

2. Do one of below processes based on your Microsoft Excel version:
(1) In Excel 2013's Format Axis pane, expand the Number group on the Axis Options tab, click the Category box and select Number from drop down list, and then click to select a red Negative number style in the Negative numbers box.
(2) In Excel 2007 and 2010's Format Axis dialog box, click Number in the left bar, click to highlight the Number in the Category box, and then click to select a red Negative number style in the Negative numbers box.

Note: You can also enter the code of #,##0_ ;[Red]-#,##0 into the Format Code box and click the Add button too. By the way, you can change the color name in the format code, such as #,##0_ ;[Blue]-#,##0.

3. Close the Format Axis pane or Format Axis dialog box.
Now all negative labels in the selected axis are changed to red (or other color) immediately. See below screen shot:

Sometimes, you may want to change labels' font color by positive/negative/0 in an axis in chart. You can get it done with conditional formatting easily as follows:

1. Right click the axis you will change labels by positive/negative/0, and select the Format Axis from right-clicking menu.

2. Do one of below processes based on your Microsoft Excel version:
(1) In Excel 2013's Format Axis pane, expand the Number group on the Axis Options tab, and then enter [Blue]#,###;[Red]#,###;[Green]0; into the Format Code box, and click the Add button.
(2) In Excel 2007 and 2010's Format Axis dialog box, click the Number in left bar, enter [Blue]#,###;[Red]#,###;[Green]0; into the Format Code box, and click the Add button.

Note: In the format code of [Blue]#,###;[Red]#,###;[Green]0; , the "Blue" means it will change all positive labels to blue, "Red" means it will change all negative labels to red, "Green" means 0 will turn to green, and you can change all color names based on your needs.

3. Close the Format Axis pane or Format Axis dialog box.
Then all labels' font color are changed based on the format code in the selected axis. See below screen shot:

If you want to change axis labels' font color when label numbers are greater or less than a specific value in a chart, you can get it done with conditional formatting too.

1. Right click the axis you will change labels when they are greater or less than a given value, and select the Format Axis from right-clicking menu.

2. Do one of below processes based on your Microsoft Excel version:
(1) In Excel 2013's Format Axis pane, expand the Number group on the Axis options tab, enter [Blue][<=400]General;[Magenta][>400] Format Code box, and click the Add button.
(2) In Excel 2007 and 2010's Format Axis dialog box, click Number in left bar, enter [Blue][<=400]General;[Magenta][>400] into Format Code box, and click the Add button.

3. Close the Format Axis pane/dialog box.
Note: The format code [Blue][<=400]General;[Magenta][>400] means if labels are less or equal to 400, they will be changed to blue, and if labels are greater than 400 they will be changed to magenta. See below screen shot: