Are your strings all the same age/brand/type ?
Maybe look at how you play them. Are you playing each string with the same force and dynamics, and using the same part of each plucking finger ?
If you put your fingers together, your 2nd finger is longer than your index, so using 2 fingers to pluck, you will get a difference in tone because of the angles and dynamics of the fingers hitting the strings. You'll find it's just a matter of practice, i.e. practice playing scales or whatever, and try to play each note the same with both fingers.
A compressor is really just to even out the dynamics of your playing, not really for the tone.