QGIS 3x setting scale-based label color

I want to have a different color for my label when zooming out.

I’ve prepared an expression, which is not valid

 CASE 
 if(@map_scale>1750, (color rgb 128,128,128))
 else
 "auxiliary_storage_labeling_color"
 end   

There is a good example here:

Setting different colors based on value when labeling using QGIS

but applies to the certain data attribute table feature.
How to write a scale-based expression?
What have I done wrong in my case?

if(@map_scale>1750,color_rgb(128,128,128),"auxiliary_storage_labeling_color")

Case
When @map_scale>1750 Then color_rgb(128,128,128)
Else "auxiliary_storage_labeling_color"
End