After hearing lots of different opinions on the best way to buy astrals (do we sell our greens or not? try selling blues?) I decided to record the purchases on a spread sheet to actually get a real answer that wasn't mired in a pitfall of confirmation bias.

First, I want to say a big thanks to Shade for helping with the data - it would take forever to get these stats without his bottomless bag of gold for astrals.


Conclusion
I'll post the data I collected at the bottom, but for those who don't want to read the stats, here is the meat of the results.

If you still need orange (or even purple) astrals, you should sell your greens. By selling greens you gain more star points and more chances to gain higher color astrals, however the trade off is a loss of xp.

If you have all the oranges you need, and don't have any holes to fill, then you should eat your greens to lvl up your current astrals.

After buying 100 astrals, if you don't sell the greens you will gain about 55 more xp but you will lose out on an extra 61 star points. Because the chance of getting a purple or an orange is so low, you don't actually gain a significant benefit from selling greens, however it is still there.


Data
Each time an astral was purchased, I recorded what lvl it was bought at, (ie Magus, Ceres etc) and what color astral was gained (green, blue, etc). Here are the results:

Magus - 371 greys, 527 greens
Ceres - 104 greys, 169 greens, 127 blues
Pallas - 20 greys, 44 greens, 85 blues, 15 purple
Satum - 13 greens, 42 blues, 27 purples
Curon - 11 blues, 7 purples, 7 oranges

% chance to gain an astral: (rounded)
32% grey
48% green
17% blue
3% purple
0.005% orange

Cost per astral: 4769 gold
Xp gained per astral: 11.74

Extra buys from selling greens (after 100 buys) : 36


Every 100 buys:
Xp gained: 1174
Starpoints: 168

Every 100 buys selling greens:
Xp gained: 1119
Starpoints: 229