Java: non-static method cannot be referenced from a static context

import java.awt.Color;

/**
 * Classe ByteGrayColor: Pour manipuler le coleur d'un pixel en niveau de gris (grey level).
 */

public class ByteGrayColor implements GrayColor {

    private static final int MINIMUM_GRAY_LEVEL = 0;
    private static final int MAXIMUM_GRAY_LEVEL = 255;
    private static final int OPACITY = 1;

    public static final ByteGrayColor WHITE = new ByteGrayColor(MAXIMUM_GRAY_LEVEL);
    public static final ByteGrayColor BLACK = new ByteGrayColor(MINIMUM_GRAY_LEVEL);

    // entier compris entre 0 (noir) et 255 (blanc)
    private static int grayLevel;

    // accesseur
    public ByteGrayColor(){
       this.grayLevel = MINIMUM_GRAY_LEVEL;
    }

    // constructeur
    public ByteGrayColor(int grayLevel) {
        this.grayLevel = grayLevel;
    }

    // constructeur du grayLevel via un double
    public ByteGrayColor(double luminosity) {
        double conversionLum = luminosity*256;
        int conversionLumInt = (int)Math.round(conversionLum);
        this.grayLevel = conversionLumInt;
    }


    // accesseur
    public double getGrayLevel(){
     return grayLevel;
    }




    // accesseur retournant la luminosité de la couleur (entre 0 noir et 1 blanc)
    public double getLuminosity(){
        double grayLevelDouble = new Double(grayLevel);
        double lumosity = grayLevelDouble/256;
        return lumosity;
    }


    /////////////////////////////////////////////////////////////////////
    public GrayColor invert(){
        return new ByteGrayColor(MAXIMUM_GRAY_LEVEL-this.grayLevel);
    }


    @Override
    public Color getColor(){
        float component = (float)getLuminosity();
        return new Color(component, component, component);
    }


    //comparaison du niveau de gris général à celui de l'objet o
    public int compareTo(GrayColor o) {
        int oGrayLevel = (int) o.getGrayLevel();
        return this.grayLevel-oGrayLevel;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (this.getClass() != o.getClass()) return false;
        ByteGrayColor color = (ByteGrayColor) o;
        return this.compareTo(color) == 0;
    }

}

public class DecreaseGrayLevels implements Transform{


    int nbGrayLevels;


    ByteGrayColor grayLevelImport = new ByteGrayColor(1);
    
    // --> erreur ici <--
    // Cannot resolve symbol 'getGrayLevel'
    grayLevelImport.getGrayLevel();


    @Override
    public void applyTo(GrayImage image) {

    

    }


}

/** 
Interface for transformations to Image
**/

public interface Transform {
    void applyTo(GrayImage image);
}

public class DecreaseGrayLevels implements Transform {

    int nbGrayLevels;

    private void toto(){
        ByteGrayColor grayLevelImport = new ByteGrayColor(1);
        grayLevelImport.getGrayLevel();
    }

    @Override
    public void applyTo(GrayImage image) {
    }
}