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This is all detailed in a question I posted in another MPBI forum: https://community.powerbi.com/t5/DAX-Commands-and-Tips/Conditional-formatting-DAX/m-p/2561478#M72884 
Posting here for more visibility.
Hi, @Back2Basics
You can try the following methods.
Matrix select =
IF (
    ISFILTERED ( 'Report measures'[Unique ID] ),
    IF (
        SELECTEDVALUE ( 'Report measures'[Unique ID] ) IN VALUES ( Workbook[Unique ID] ),
        1
    )
)
Color = 
IF([Matrix select]=1,"Green")Select the background color of Linked ID and set Color measure.
Is this the output you expect?
Best Regards,
Community Support Team _Charlotte
If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.
Thanks so much for the reply @v-zhangti 
Unfortunately what I'm trying to do is slightly different. The table I have oes not display the Linked ID field, but a field called 'outcomes'. This is setup as a matrix and counts the outcomes against impact level, with the count being number of ID's. 
I'm trying it a different route now but having errors when merging which I can' figure out either: https://community.powerbi.com/t5/Power-Query/Merge-not-working/m-p/2563769 
 
					
				
				
			
		
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