If you have a good relationship with your college roommate, and you have finished your studies and joined the work, it is totally acceptable to attend their wedding. If you are in the same city, or it is easy to arrange time, then attending a wedding is also a good way to celebrate their happy event and relive the old days.
However, if your college roommate relationship is relatively average, or your own personal situation does not allow you to attend the wedding, then you can choose to politely refuse. In this case, you can express your blessing by sending a text message or calling them and explain why you can't attend the wedding in person. If you are worried that they will be disappointed, you can also put forward some alternatives, such as arranging parties or dinners before and after the wedding.
In short, whether you attend the wedding or not, you should be polite and respectful, and express your blessings and support in an appropriate way.