can you have a look at this link 

https://collaborate.pega.com/discussion/repeating-grid-record-color-change-respective-page-property

Kindly try this Inline css 

<pega:when name="whenRule">color: red;</pega:when>

have you tried creating your own derived label property, you may need to write javascript function as well which will return random color value.  

I think we may not do this by OOTB controls, we might need to have a custom control, inside which we can set the random color for the label.