Using Pinhole Designer with a user constant of 1.4 instead of the default 1.9 and a light wavelength of 500nm, the suggested diameter for a focal length of 100mm is .313mm. The optimum diameter has been argued for well over a hundred years, and isn't that critical for most photography. For macro pinhole photography, the diameter should be reduced even further

I haven't noticed any reciprocity failure in 10 minute enlargements, but any failure might be masked by rather casual evaluation of prints.

As for color separation in pinhole photography, it might be present, although masked by the normal unsharpness of pinhole images. Pinholes do have very slight chromatic aberration.

My technique for making pinholes is similar to edp's. A very hard smooth stone is ideal for the smaller pinholes. For the smallest ones, I keep an ordinary sewing pin in the hole while stoning down the dimple. This gives more control over the pinhole diameter and leaves less of a burr than stoning the dimple without the pin. A sheet of .002" brass shim suffices for many pinholes, and seems to work smoother than aluminum cans. Thinner shim stock is fragile, and thicker shim stock requires more effort to make pinholes with thin edges.