Refrigerant dyes mix with oil and the oil mixes with the liquid refrigerant. As the refrigerant evaporates, the oil/dye is carried by the vapor as an entrained mist. The oil will migrate to and thru the leaks, carrying the dye with it. To use the dye you need to add it and run the system long enough for it to circulate. But if the leak is significant, you may see oil even without the dye.
You can do a quick visual inspection of the plumbing, connections, fill ports condenser and compressor. If you see oil (dyed or not) at any of these points, that's where your leak is. In particular, check the compressor, between the clutch pulley and housing for signs of oil, indicating a shaft seal leak.
If the shaft seal is OK, the next most likely culprit is the evaporator. You need to do some disassembly to be able to see the evap. Most leaking evaporators have been doing so for quite some time....(hmmmm no cold air...let me add some refrigerant....then repeat next spring...) and there is often an obvious build-up of oil on the core by the leak and in the bottom of the housing.
Hope this helps.
Tedster