Background Color the events on calendar based on a picklist value

What your code says is to use orange for Corp - Workday Rising US, blue for Corp - Elevate US, and green for anything else. The specs are not the same.

Is the formula in a UI-Only field in the model used as the Event Source on your calendar? Is that field used in the Background Color property on the Event Display tab for the Event Source?

Your code does have a syntax error as a ui-only field formula:  the first close-parentheses, after ff9b69", should be at the end, after 217a37"). The format is:

if ( a, then,
 if ( b, then, else )
)

The way I read your specs, you could code the formula as 

if ( {{Campaign_Channel__c}} = “Corp - Workday Rising US”
  || {{Campaign_Channel__c}} = “Corp - Elevate US”
  || {{Campaign_Channel__c}} = “Corp - Exec Programs”
  || {{Campaign_Channel__c}} = “Corp - Tradeshows”,
“#217a37”,
“#40a0ff”)

(Also, your ‘blue’ value 40a0ff needs a # in front of it.)

I think your Formula is invalid, try this instead:

IF({{Campaign_Channel__c}} = “Corp - Workday Rising US”, “#ff9b69”,
IF({{Campaign_Channel__c}} = “Corp - Elevate US”, “#40a0ff”,“#217a37”))