I had this problem too, I noted down that I had a red/green colour deficiency.
First at the doctors physical they did the usual Ishihara test (the dots arranged in a circle) which I failed (the doc said I was very colourblind

), then when I went for my interview I first had to do the Trade Colour test (or whatever it was called) since I am going in as an ET(ME).
Basically it was a set of wires, with a matching pair on either side. Around 15 different wires, each with a different colour, and some with bands of other colours, such as red with green stripes. You have to match the wire on the left with the corresponding wire on the right, and they have a little connector that they plug into. I had 3 minutes to do the test (which was plenty of time to do it and check it over 3 times). I checked it thoroughly as any mistakes at all result in a fail, and therefore not be able to join up. Honestly the wires are very easy to tell apart, there was nothing to try catch you out (nothing like multiple slightly different shades of red, it was plain red, plain yellow, etc) and the recruiters are very sympathetic and understanding, they leave the room so there is nobody watching, don't stress about it.
The most difficult part was trying to get the finicky little wires to behave themselves and cooperate
I really would not worry about it, it was fairly easy, and as someone who completely failed the Ishihara test, it was a big relief to pass it.
Please note I am not a recruiter, just a recruit who went through this process about 2 months ago. I am not sure about the process for any other trade though, but I assume most of them would be about the same