Accessing colors in a resource dictionary from a value converter

I defined several colors in a ResourceDictionary. e.g.:

<ResourceDictionary ...>
  <Color x:Key=Gray1>#FFF7F1F3</Color>
  <Color x:Key=Gray2>#FFDDD8DA</Color>
</ResourceDictionary>

So I can reuse them everywhere in application.

Now I wrote a value converter to convert the items inner state to the related color.

How can I access the defined colors in the code of the value converter?

My first thought was to pass the dictionary as converter parameter. But I don't know how to achieve that.

Regards

EDIT

Application.Current.Resources is not an option. Because I won't have access to it later.