In American English, this part is simply called the link. It is part of the steam valve control gear, in this case a walschaerts valve gear, invented by Edige Walschaerts of the Belgian state railways. The link is rocked by the eccentric rod, driven by the eccentric. It transmits this motion to the radius rod, which, via further monkey motion, drives the steam valve. The end of the radius rod can slide up or down on the arc shaped faces of the link, and its position is determined by the engineer, using the reversing handwheel. In exteme up or down position, the cylinder gets maximum steam for full power ahead or reverse. Centered, the radius rod does not move at all, neither does the locomotive. The reason for this little treatise on steam engineering is to answer your paint question. The link should be painted the same color as the rods and valve gear, except that the faces that the radius rod slides on are polished steel, well greased.