Dear List,
Indeed Vallabhadeva (C10th) understands the birch-bark to be red because of being written upon with vermilion or the like:
dhāturasena sindūrādidravyeṇa nyastākṣarā likhitavarṇāḥ/ ataś cākṣarāṇāṃ lauhityāt kuñjarabinduvac choṇā lohitāḥ/
But it is perhaps conceivable that they are red because they have been sealed with red sealing wax, which is what is suggested by this echo in Dhoyī's Pavanadūta:
āttaṃ karṇāt praṇihitapadaṃ sāñjanair aśruleśaiḥ
baddhaṃ tāpaglapitabisinītantunā bandhanena/
yatra strīṇām adhararucakanyastasindūramudraṃ
tālīpatraṃ praṇayini jane premalekhatvam eti// 40
James Mallinson's translation reads:
Where palm-leaf decorations taken from the ladies’ ears,
inscribed with teardrops mixed with mascara,
bound with ties of lotus fibres withered by the fever of separation
and sealed with vermilion lipstick, become love letters to their sweethearts.