You don't need to change the value of the style property itself, you need to press the Add button in the top right corner of the panel with all the properties, then access the property within the style object. So for yAxis title color do yAxis.title.style.color and then pass the value as you would pass for other color properties. It can be a keyword, can be a hexadecimal:
Changing style of HTML5 chart title text, tooltip text, legend text, etc.
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You don't need to change the value of the style property itself, you need to press the Add button in the top right corner of the panel with all the properties, then access the property within the style object. So for yAxis title color do yAxis.title.style.color and then pass the value as you would pass for other color properties. It can be a keyword, can be a hexadecimal: