OpenLayers pass character parameters problem

I want send character parameter (‘GREEN’) to GeoServer as following:

var untiled = new ol.layer.Image({
    source: new ol.source.ImageWMS({
      ratio: 1,
      url: 'http://localhost:8080/geoserver/newyork/wms',
      params: {         
            "LAYERS": 'newyork:viewStation',
            "viewparams": 'col:'+'GREEN'
      }
    })
  });

viewStation is a SQL query:

SELECT * FROM station WHERE color=%col%

but the code doesn’t work, I also tried ‘col:GREEN’

What can I change?