I am going to assume this is a WinForms questions (which it feels like, based on it being a "program" rather than a website/app). In which case you can simple do the following to change the text colour of a label:
myLabel.ForeColor = System.Drawing.Color.Red;
Or any other colour of your choice. If you want to be more specific you can use an RGB value like so:
myLabel.ForeColor = Color.FromArgb(0, 0, 0);//(R, G, B) (0, 0, 0 = black)
Having different colours for different users can be done a number of ways. For example, you could allow each user to specify their own RGB value colours, store these somewhere and then load them when the user "connects".
An alternative method could be to just use 2 colours - 1 for the current user (running the app) and another colour for everyone else. This would help the user quickly identify their own messages above others.
A third approach could be to generate the colour randomly - however you will likely get conflicting values that do not show well against your background, so I would suggest not taking this approach. You could have a pre-defined list of "acceptable" colours and just pop one from that list for each user that joins.