Your 7.5 w lamp is Very red/yellow to begin with. Even a 100 w bulb puts out something like 2800 k (don't have precise data available now). As the wattage diminishes, so too the temp, and you are getting into serious red territory. Probably part of the reason you get longer times, because a proportion of the light output just doesn't count. What light does count is probably pretty much equivalent to a 00.