Summary: If you have, some feelings for your office colleague then write her a letter. You can speak out those things, which you have in your heart but you cannot express them. She is your colleague so you have to careful and much clear with your words and emotions. If you want some sample of romantic letter to colleague then here it is.
To, Ms. Nidhi
E-2/38 Rohini Sector-11,
You might be thinking why I should write a letter when I am working with you sitting in same corner. This is because not all can be expressed directly with spoken words. Though we are working for past 6 months, I still remember the first day I saw you when your voice attracted me. Sincerely I am impressed with your intelligence and hence think about you always and talk about the same with everyone who comes in my way.
With the good impression, I came with an invitation from my side to come for dinner tonight. I am sure you might not have expected this and might not be ready to come for dinner. I do not compel that you should have the same expressions for me. So just, let me know what you have in your heart. I would wait for your reply with pleasure.