If you are happy to play around with Scalable Vector Graphics then the Scalable Vector Graphics - Custom Visualization app might be something to consider. There are a number of examples within the app that show parts of images changing colour based on numeric thresholds.
Can the icons change color? Yes. Easy? Depends on a lot of detail you haven't provided.
- You can look into the Dashboard Examples App for ideas: https://yoursplunkaddress/en-US/app/simple_xml_examples/contents
- My go-to might be this one https://yoursplunkaddress/en-US/app/simple_xml_examples/simple_single_color
- or this one https://yoursplunkaddress/en-US/app/simple_xml_examples/custom_decorations
- You can use custom CSS with tokens to do it. Look into http://dev.splunk.com/view/webframework-developapps/SP-CAAAEQA and https://docs.splunk.com/Documentation/Splunk/7.3.1/AdvancedDev/UseCSS
Once you have a better idea of the question you want to ask, post another question to Splunk Answers saying what you have now and what you have tried so far.
If they are icons (images) then find that image in
<<YOUR_APP>>/appserver/* folder and replace it with your client's image. Just make sure about the size and pixel same as old image.