Describe the bug
Achievement progression shows that necessary triggers hit:

"spectrum:progression/colored_saplings/unlock_brown_sapling": {
    "criteria": {
      "has_vegetal": "2022-01-01 16:00:46 +0000",
      "collected_onyx_shard": "2022-01-01 19:29:36 +0000",
      "collected_amethyst_shard": "2021-12-30 17:03:37 +0000",
      "collected_citrine_shard": "2022-01-01 06:32:13 +0000"
    },
    "done": true
  },
"spectrum:progression/colored_saplings/unlock_black_sapling": {
    "criteria": {
      "has_vegetal": "2022-01-01 16:00:46 +0000",
      "collected_onyx_shard": "2022-01-01 19:29:36 +0000"
    },
    "done": true
  }

However, recipes aren't showing in REI nor in Colorful World manual as unlocked.

To Reproduce
Suspect that when acquiring onyx shard from fusion, the unlock triggers didn't perform entirely?

Expected behavior
Upon acquiring the prerequisite items, saplings recipes should be available.

Screenshots
If applicable, add screenshots to help explain your problem.

What is required for that advancement? I have constructed the multiblock (says 'complete'). Is that what is not registering properly then?

Depends on what version of Spectrum you are running on. Are you on 1.2.0 Midgame1 or higher?
In versions before there is a blocker at this point, because lots of features were not done yet.
Otherwise just clicking onto the pedestal with the Onyx structure should do it.

Yep, you replied literally as I was checking. I assumed (incorrectly) that because the pack was added to AoF5 recently, it would have been a more up to date version of your pack. Sure enough, it is only earlygame4... explains so much.
Thanks for the time to reply. Enjoying your mod so far!

No problem at all! Have fun :)

For the recipe to unlock / be craftable you also (while now shown) need the advancement "Construct the Spectrum Temple". Do you have that one?
Like the crafting of the CMY Pedestal it is considered a major milestone, unlocking a new recipe tier, that these need.