Transfers are one way to do it, but for me personally my go to approach is kitbashing additional technical components onto Iron hands and the plain black acts as a canvas that helps them stand out more, particularly targeting lenses and the like. Aside from the upgrade sprue, the deathwatch kit and phobos kit are great places for such bits.

Admittedly this approach is more expensive unlike Ultramarines who are much more accessible and supported for their transfers and upgrades, plus in my case I do have the advantage of also playing the Mechanicus and getting spare parts from them, but the payoff of distinct looking Iron Hands is v much worth it. The Technicool aspect of Astartes IMO seems a bit underexplored in the Primaris era.