As @NetMage mentioned, you need to make all Members of Culori public. Also you need to create an instance of your Culori class to access the members.
var culori = new Culori();
var myColor = culori.DEFAULT_COLOR;
Alternatively you could make all members of Culori static:
public static Color DEFAULT_COLOR = ColorTranslator.FromHtml("#FF0000");
Then you should be able to access it without creating an instance of Colori:
var myColor = Culori.DEFAULT_COLOR;
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solved Use colors from other file