var myCols = app.activeDocument.colors;  
for ( var i = 0; i < myCols.count(); i++ )   
{  
    if (myCols.item(i).space == ColorSpace.CMYK)  
    {  
        var oldVal = myCols.item(i).colorValue;  
		if ((oldVal[0] > 20 && oldVal[0] < 70) && (oldVal[1] > 20 && oldVal[1] < 70)  && (oldVal[2] > 20 && oldVal[2] < 70) && (oldVal[3] > 20 && oldVal[3] < 70)) 
		{ 
			var newVal = [66, 66, 66, 0];  
			try  
			{ 
				myCols.item(i).insertLabel('oldColorValue', oldVal.join()); 
				myCols.item(i).colorValue = newVal;  
			} catch(e){}  
		} 
    }  
}