In the times when Paul was writing, slaves did not have rights - they just kept out of trouble and received some benefits by doing their duty.  Paul knew that there were many slave-masters in Colossae, and some of them had come to believe in Jesus Christ.  Philemon was one of them (Philemon 1:8-16).  Interestingly, the Apostle does not challenge the concept of slavery as one of the social habits of the day.  Instead, he challenges the immoral habit of treating the slaves like dirt; like an object or a possession ... in the same way that he challenged the assumption that it was acceptable to treat women harshly (Colossians 3:19).
 
The moral basis for good employment relations is that every boss has a boss!  God is the master of everybody including the employers.  They must give an account to Him.  He is kind and fair to them, so they should be caring and fair to their slaves.  If they need help, or equipment or food or clothing or any kindness, it must come from the manager.  They should be the provider in the same way that God provides for them.  It is also a workplace application of Jesus' words in Matthew 7:12: "So in everything, do to others what you would have them do to you, for this sums up the Law and the Prophets."

Who should determine what is “…right and fair …”?  This is an important question because human history is littered with the legacy of cruel despots who sincerely believe that they have the right to force people to obey them.  The barbarity of angry, proud people who despise their neighbours, and even their family (Genesis 4:1-8), cover the record of humanity from its earliest days until now.  The answer is that only God has the right to define what is right and fair: He is our Maker, Sustainer, Ruler and Judge (Acts 17:24-31).  That is why His Word is the instruction manual for all in leadership and His Son is the ultimate example of how to do what is right.  Without humble attention to the Bible and submission to Jesus, God’s authority comes second-hand at best, or is ignored at worst.
 
Workplace cruelty is still far too common.  That is not to say that workers ought to take advantage of their employers, or that the boss should not be clear about what is required, to have a strongly focussed workforce. But abuse is not acceptable.  Now, note that this verse is not addressed to the workers; it is not intended to stir up dissent or be a bargaining tool for better pay or conditions.  Peter says, "Slaves, in reverent fear of God submit yourselves to your masters, not only to those who are good and considerate, but also to those who are harsh." (1 Peter 2:18).  No! This is written to the masters.  They have to settle their own conscience before the Lord.  So, if you have any management responsibility or you purchase goods or services, remember that the Lord is your 'Boss'.  He is watching to see how you measure up to His standard (Luke 6:29).  He wants to see you demonstrating His character as you deal with other people.  So do a realistic appraisal of yourself; identify your weaknesses, and change!  It will be for your blessing, the good of your workers and the glory of God.