This lamp don't lights at the moment that the power applied to it, but there is a delay of a sec or less (Without electrodes glowing), then a thermionic discharge, guardually but fastly appears at the electrodes and then the lamp ignite. This is more typical of a warm starting, and this lasts about 1 1/5 secs max.
However i recently discovered that as my lamp finally ignite and lit, a blue glow commonly appears on the electrodes for a ~1/4 sec, and this can only happens, if the lamp ignited with cold cathodes (Instant start).
Why this is happening?
Is this truly a warm starting?
Lamp date code: bst428

This may happen on the programmed start ballast, when the preheating voltage/current are not sufficient, so electrodes do not reach the required temperature and/or their emission layer is worn out or damaged in another way (so require higher temperature to emit electrons and/or the heat transfer from tungsten core onto the coat surface is poor).
Same effect may happen on voltage mode heated electrode, when the filament is broken, but it's coat is still in good condition (e.g. result of mechanical shock; this happened to me while i was debugging my dimming ballast and the only visible problem was poor arc stability at low power setting - i thought the control loop dynamic stability was at fault, but i discovered the real cause later on)

Electrodes do not have to glow visibly, in fact the visible (incandescent yellow and/or pure thermionic emission white) glow indicate the temperature is too high (however this is considered of less problem then insufficient heating). At the generally recommended preheat temperature (so the filament resistance is 4x cold resistance) the incandescent glow is very dim red, so practically not visible (unless in dark room).

Do not forget, then good electrode coat is of white color, so it does emit the (incandescent) light only very poorly.