I am not sure where you are looking for trains, but there are trains before 11 am. There should not be any significant reduction on train services on 1st May, holidays have the same trains as on Sundays.
I looked on the DB website: https://www.bahn.com/en/view/index.shtml
There are two ways to do this
1) Train to Saint-Louis (last station in France), then bus every half hour to the airport.
2) Continue on train to Basel. then bus every 7½ minutes to the airport.
For option (1), the DB site gave me the following trains:
Colmar 07:23, direct, St-Louis (Haut-Rhin) 08:12, bus depart 08:20
Colmar 08:28, change Mulhouse, St-Louis (Haut-Rhin) 09:42, bus depart 09:50
And the bus timetable from the station to the airport is here: http://distribus.com/uploads/ligne11.pdf
If you don't like the infrequent bus connection, the options via Basel are:
Colmar 07:23, direct, Basel SBB 08:20, bus departure at 08:20 08:27 08:33 08:35 etc.
Colmar 08:28, change Mulhouse, Basel SBB 09:50
And the bus is on your left as you step outside the station. It is a frequent shuttle, there is always a bus waiting. Bus departure times here: https://www.bvb.ch/wp-content/bvb/Dokumente/Fahrplaene/Haltestellen/2018/Linie%2050/ah_02050A_n18_a_00007.pdf Bus takes 17 minutes to airport.