Honestly I asked the same question. I am now advising to wait till day 17 and check from top to bottom and if you only see an air cell your good to go lol. Brown eggs suck to candle. I did however buy a new super bright led flashlight. It blinded me in the store. It was at lowes and was a coastline tacticle flashlight. It has 108 lumens. I hope to have better luck this time around.
I bought this one from Lowes. It is model # SL5W09L. It has 192 lumens. It has a rechargeable battery that they claim will last the life of the unit. I paid $40 +tax. That's a little high for this purpose. But, I can get many other uses out of it. I made a little cone from card stock and taped it with several layers of painter's tape. The smaller end is 1 1/2". I don't have any candling pics yet, but, I candled my brown Buff Orpington eggs on day 5 and a couple on day 6. I could easily see veining and an eye. I would think any LED flashlight with high lumens would work depending on how dark and thick your eggs are. There are a lot of options. Good luck to you.
all my eggs are brown, and i use a small standard flashlight. usually on the real dark ones it takes till about day ten to see anything, but usually around day 5 i can see in them.