Good question,
In the early days, ie during the uprisings, it seems many or most of the volunteer units carried flags, very often red with the white eagle.
Most of the Poznan army carried regulated regimental flags and standards - also in the field.
When it come to units outside the German controlled areas, it becomes more murky. There are less photos, museum pieces, documents and such - and after 1945 the now White Russian and Ukrainian areas were de-polonized - also historically - and so information is difficult or impossible to find.
The 8. Lancers (an "Austrian" regiment) carried a standard, made by the officers' women, but sent it back to depot, when the fightings against Budionny started. It was carried 1939 but since lost, do not remember burried or burned by the Poles.
The Poles had a rule, so it is claimed, if a regiment lost a regimental colour, it was to be disbanded and its numbewr stricken from its ranks. So you see no Polish colours taken by the Germans in 1939. Only a mere trumpet banner could be presented to Hitler in Warsaw as a "taken Polish colour".
All early colours used 1918-21 should be sent to the Military Museum in Warsaw, when the new regulation colours were presented, most of them after the war, even if some also were presented before the war ended. Not all were sent to the museum, some went to regimental or local museums. It has not been possible to find any lists, articles or such on the subject.
But there must have been lots of locally produced colours and flags in use. So the Army Command tries to get a little order in it all through regulations, when complaining about all the different colours and flags presented and given to any passing units by communities, towns and societies. Many units so getting more than one flag, and of course each in its own style.
I have a single photo of even company colours used in the field 1919-20.
So you will have to use your imagination on how the colours of 26pp can have been.
26pp was formed around the towns of Radom, Końskie and Włoszczowa.
I am rather sure, they have had something, not least as it was one of the very early formations and with close ties and financing from the Polish independence movement, the former Legions and POW in Galizia.
Red with white eagle on the one side and on the other some badges or descriptions showing the local area or towns, a Madonna in a field and some text, anything goes.
The Polish Legion, when figthing for Austria 1914-15 did not get any official Austrian Army colours, but they certainly had their own Polish, and that a very elaborate of which photo exists from 1915.
The 26pp did first get an official, regulation colour from the grateful local communities in Lwow on September 17, 1922.