There are at least two theories on the origin of Solothurn's colours.
One holds that they came from the vexillum of the ancient Roman
Legion of Thebes which was massacred in Helvetia in 302. St. Ursus,
the patron saint of Solothurn, was allegedly an officer of the
legion. Less likely, Solothurn adopted the colours of Austria (as
did Schwyz and Unterwald).
According to legend, Ursus was an early Christian who proselytised
the Solothurn region. He was persecuted and eventually decapitated
on the Aare River bridge, and his body thrown into the river. But
the body swam ashore, picked up its head, and walked to its burial
place. Thus did Ursus become the patron saint of the city. The seal
of the city has always depicted St. Ursus, and as early as 1394 the seal
contained a red and white background identical to the current flag,
so these are regarded as St. Ursus' colours. Solothurn became an
independent state in the empire in 1447, and was admitted to the
Swiss confederation on the same day as Fribourg in 1481. This was
done over the objections of Obwald, whose flag at the time was
Simple rectangular cantonal flag, as shown in Kannik (1956). Ole Andersen, 4 August 2002
Flaggen, Knatterfahnen and Livery Colours
by Pascal Gross
Flaggen are vertically hoisted from a crossbar in the manner of gonfanon, in ratio of about 2:9, with a swallowtail that indents about 2 units. The chief, or hoist (square part) usually incorporates the design from the coat of arms - not from the flag. The fly part is always divided lengthwise, usually in a bicolour, triband or tricolour pattern (except Schwyz which is monocolour, and Glarus which has four stripes of unequal width). The colours chosen for the fly end are usually the main colours of the coat of arms, but the choice is not always straight forward.
Knatterfahnen are similar to Flaggen, but hoisted from the long side and have no swallow tail. They normally show the national, cantonal or communal flag in their chiefs.