I've gone through gallons and gallons of epoxy. A number of time I buy the 5 gal. pail.
The major failure elements for people using epoxy are:
-poor mix, ratio and stirring.
-improper application.
-excessive squeeze-out.
Epoxy is a two part chemical mix. Therefore it is critical that the two parts be mixed in the correct ratio, and mixed well to ensure that all the mixture has been combined equally.
The epoxy will wick into the wood fiber. You need to ensure that there is epoxy in between the two pieces of wood that you will be bonding. That is why you prime the wood surfaces first, you want to keep a wet look prior to sticking the two pieces together.
If you do not have enough epoxy coating the mating surfaces, or you over clamp the joint squeezing out too much epoxy, you will starve the glue joint. While it may seem to be bonded, only a certain percentage of the surfaces will actually be bonding. This is one reason, actually the primary reason, why you use a filler in the epoxy when bonding, it ensure more contact and makes it harder to accomplish excessive squeeze out. There are a variety of fillers. The most common for structural bonding are silica and cotton fiber. Wood flour from your sander is also excellent.
Yes, the hardener does take on an amber tint when its exposed to oxygen. The extent of it depends upon how full your container is. A little hardener in a big can/jar exposed for a longer time will take on more colour. Remember, the epoxy is still used 1 part hardener to 5 parts resin, so the intensity of colour in your mix is small. Even when used as a film coating, it is pretty much overwhelmed by the wood colour anyway, you'd be hard pressed to account for any colour in the result as due solely to the epoxy. In a glue joint, the colour should be a non issue. There will be a greater impact from having any epoxy glue line there at all, than can be attributed to the tint of the epoxy.
Speaking of glue lines, I have sometimes used this to effect by really adding some colour. Adding red (or other colour) resin pigment in a stack of pale wood laminates can have a striking effect. I will play with that again when opportunity strikes.
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