SSRS Custom code error

Question

I am using a custom code for text box background which I got here :

https://blogs.msdn.microsoft.com/davidlean/2009/02/16/sql-reporting-how-to-conditional-color-14-the-basics-report-expressions-custom-code/

However when I want to use it in an report expression I get a red error line under the function name. I am using Visual Studio and VB.NET.

What am i missing? I have pasted the code in the code under report properties.

Regards

EDIT :

I have used the following code as an expression : =code.ColorRYG(Fields!BlokkeklaarPersentGelewerKultAliasnaam.Value,Fields!BlokkeklaarmaksimumKultAliasnaam.Value,Fields!BlokkeklaarminimumKultAliasnaam.Value,0) A red squigly line appears under ColorRYG. The code I have pasted (Report Properties Code) is as follows :

.mobile-folder ul {display:none!important;}


.mobile-folder ul.expand {display:block!important;}

Thank you.