The only through Colmar-Paris trains in the May 1976 timetable were the night train via Strasbourg on weekdays or the 17.56 on Fridays, which would probably have been full of conscripts going on weekend leave. Best possibility is 12.06 Colmar-Strasbourg 12.43, then change to Train 1006 13.40 Strasbourg-Paris Est 17.37. 1006 comes from Salzburg, so if it is late or your protagonist wants to get away quick, there is a 12.50 from Strasbourg (Train 1606) that stops at a lot of intermediate stations and reaches Paris Est at 18.02.
Via Mulhouse it is 11.31 departure and arrive Mulhouse 11.55 (train starts from Luxembourg, so could be late) and change, then Mulhouse départ 12.16 on Train 1044 and arrive Paris Est 17.18. The next Mulhouse-Paris is not until after 16.00.
A personal opinion from someone who lived in Colmar, albeit some years later, is that travellers would automatically default to going via Strasbourg, rather than via Mulhouse.
Two notes to the above: the times I have given are from May 1976 and I can’t be sure whether they ran as shown after the end of the month, although the 13.40 Strasbourg appears to. Also, I do not have the times of most of the Mulhouse-Colmar-Strasbourg local trains, which might give another option for getting to Strasbourg.
Source: Thomas Cook Continental Timetable, May 1-29 1976.