You can't have 2 different colors on the same text field. The caption can only be one color, as can the 'show title in chart' text available on the general tab of the chart properties.
I would suggest using text boxes , potentially overlaid over the caption to create 2 different pieces of text that are vertically aligned by are colored differently
Here the red is in a different text box. The layout tab / layer property is set to top and the general tab / transperency is set to 100%. I kept the borders on to show the box below, but definitely remove them (also layout tab)