Attempting to locate every possible colour combination for Mexico City 1968 continues to be a Herculean task.  Not only was a different base colour used for every day of the events, but a different coloured strip was utilise to identify the price band of the ticket. There are therefore either 28 or 35 tickets required to complete the set (I haven't yet been able to determine which).

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Rear of ticket

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Close-up scan

Notice the repeated 'Mexico 68' logo, repeated upon the ticket's rear.  Only visible when looking closely.