I think there are issues using levels to calculate the ratio between xp and stats instead of directly calculating ratio between xp and stats. For example level 7 orange attack, which cost 1728 xp total, if i were to invest all that xp into blue, i would get a level 10 blue with 1776 xp total, now going to the stats, level 7 orange has 87+(26*7) = 269 attack, a similar EXPERIENCED blue would give 21+(12*10) = 132 attack, thats quite a massive difference, as i see it no reason to be using lower colored chakra if a higher one is available.