Enhanced Scatter 2.0 - cannot use a measure for Customized Colour field?

I've built a traditional heat map to represent Risk impact x severity. I wish to make it dynamic so the mitigation selected (Inherent / PreMit / PostMit) is reflected wihtin the visual. It works fine for the size of the bubble (sum of cost) but it won't accept my measure for the 'Cutomized Colour' field. It works fine when using a calculated column, but i need real-tim change for the user to compare not to wiat for the underlying table to be recalculated. Is this a bug or am I doing something wrong?

Worth adding the measure works fine in a Table to drive the conditional formatting of the cell depending on the filter applied.

.test Dynamic Heatmap EMV Bubble Colour = 
IF (
    SELECTEDVALUE ( Risk__Risk_Assessment_Rating__c[Risk__Mitigation__c] ) = "Inherent",
    MAX( Risk__Risk_Assessment__c[Risk__Inherent_RatingHexHash]) ,
    IF (
        SELECTEDVALUE ( Risk__Risk_Assessment_Rating__c[Risk__Mitigation__c] ) = "Residual",
        MAX ( Risk__Risk_Assessment__c[Risk__Residual_RatingHexHash] ),
        IF (
            SELECTEDVALUE ( Risk__Risk_Assessment_Rating__c[Risk__Mitigation__c] ) = "Target",
            MAX ( Risk__Risk_Assessment__c[Risk__Target_RatingHexHash] )
        )
    )
)