Use [ ] around the group name rather than ' '. using [ ] will give red lines , it is a bug from Qlik View end.
Please attached with two dimensions.
Product ID = Red
Salesman ID = Green
drill down.qvw 166.2 K
There's quite a lot going on here, so let's start with what works for me, then you can stop there if you like!
Here is the colour expression:if(getselectedcount(RegionName)=1
1 Rename the colour columns to be distinct, otherwise you get a synthetic key and possibly confusion and a loop in the data model. I have left out the alpha, but the principle is the same.
2 Name the key field in the CountryColors table so that it associates with the country in the rest of the model, presumably CountryName.
load * inline
load * inline
3 I have assumed your drill dimension is based on fields RegionName and CountryName. GetCurrentField come from QlikView and isn't part of Qlik Sense, but we can use GetSelectedCount instead. We get to CountryName when one and only one region is selected, ie getselectedcount(RegionName)=1. You need to reference the raw field name rather than the master item name. If your fields have non-alpha characters, eg space, put  around them. This isn't required in the load inline statement.
4 I don't know what RgbRegion is, but the syntax in your example would be for a function. There is no RgbRegion function in Qlik Sense. It doen't work if RgbRegion is a master item. If it's a variable, you must define it without an '=' at the start, then reference it as $(RgbRegion) in the expression.