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You have declared (as instance variable) a:

JSlider redSlider
and you have created an instance of JSlider with
redSlider = new JSlider(JSlider.HORIZONTAL, 0, 255, 0);

then you have declared...

First, this is not a compiler "warning" but an exception at runtime. Second, please learn how to interpret the stack trace, it says:

at colorslider.<init>(colorslider.java:36)

What is there at...