I am trying to add a color picker property to a block class and I have been trying to figure out how to use the specialized property PropertyColor but I keep getting errors related to property definition type. Here is what I have tried so far:

[BackingType(typeof(PropertyColor))]
public virtual String BackgroundColor { get; set; }

Error: "The backing type 'EPiServer.SpecializedProperties.PropertyColor' is not a registered PropertyDefinitionType"

public virtual PropertyColor BackgroundColor { get; set; }

Error: "Type 'EPiServer.SpecializedProperties.PropertyColor' could not be mapped to a PropertyDefinitionType"

[BackingType(typeof(PropertyString))]
public virtual PropertyColor BackgroundColor { get; set; }

Error: "The property 'BackgroundColor' on content type 'BannerBlock' is of type 'EPiServer.SpecializedProperties.PropertyColor' that cannot be backed by PropertyString, it only only supports types String."

Please help me figure out how to implement this property type so I can use the color picker.

Thanks in advance