501.7 is an argon line. I haven't heard of any commercial tubes with optics specific to this line. It can be seen in the "rainbow" from a multi-line and isolated with a filter.
501.7 is the second from the "green" end.
This is one of my favorite pictures. In it you can see the combined laser beam color of a multi-line argon, in comparison to the isolated "pure" colors in the beam
A bit of a silly poll, as there are a lot of nice laser colors, I vote Blue, but actually like my much higher powered Violet lasers best. I suppose if all colors were of equal output power and there were no cost differences, I might choose somewhere in the blue/ greens, like 501.7 perhaps, but still, that 405 to 473 range is all highly attractive IMHO. :