Typically an A/C leak will be a florescent green because the oil is dyed to make leaks easier to detect. It is also easily washed away with water and a system that has been empty for some time can make a leak very difficult to see. The best way to find a leak is filling the system. I would suggest bringing it back to where the repair was done and have them refill the system and see if a leak is present from something related to what they replaced or something they missed.