Getting 6mm T&E into 20 mm conduit would be hard, it does not draw well, personally I would use SWA cable then no need for conduit, in the main any cable for outside is black as this resists suns rays better than other colours.
There are other tough cables that can be used outside, however SWA is common so also tends to be cheaper than other cable because so much is made. However really needs to terminate in a metal box and needs glands.
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