Lyrae, there is no there is no way to upgrade the grey stats to yellow stats aside from leveling the pet.
From what I can tell, the stats that are grey are the addon's "best guess" for what the stats will be when the pet is level 25. In this case, the best guess for the Pandaren Fire Spirit is 1400 health, 289 attack power, 289 speed. This guess is actually correct, but with level 1 pets there is apparently some uncertainty for certain combinations of stats. If the stat is in yellow, the addon isn't "guessing"; it knows what the stat will be.
I don't know why the addon has to guess for certain pets, but those guesses are usually accurate. It likely has something to do with the intersection of pet families with breed types. For example, the P/S Pandaren Fire Spirit has grey stats at level 1, but the P/S Azure Crane Chick has yellow stats at level 1. It's worth noting that their level 25 stats are different from each other.
When a pet with grey stats gains at least one level, the uncertainty for that addon is resolved, so it should no longer display any stats for any pets over level 1 in grey. I tested this on my own Fire Spirit. The stats were grey at level 1, then yellow after one battle where it went up to level 5. The predicted values of the stats at level 25 did not change.
What you are observing is a just how the addon handles its predictions. The coloring of the text has no effect on game-play or whether those stats can be upgraded or otherwise changed by player.