There is no way of custom colouring the fill area under a line chart using an expression, you can change the line but that's it.
There are a couple of default colour schemes you cannot change and a min/max range you can play with under the custom colours by measure option however they are not customisable enough for you to achieve what you listed above. The default for area colour by expression is a light grey.
I get the same result as you (line is coloured, not the area).
Other options could be either colouring by Measure (not too customisable but the idea is there) or if you want specific colours as in your dashboard, to (as you suggested) use another chart (i.e. like the bar chart I have below).