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MACRO ICON TO DARKEN /GREY-OUT IF TALENT INACTIVE
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When using as your default spell-button a macro instead of the basic talent from spellbook, swapping build to one without that talent causes the tooltip-icon to reset to question mark 
instead of just darken /dim like default one does.
Wherever you’re looking for solutions, you get the same bunch of answers:
- not possible
- there is no way to have the icon on your actionbar for an ability you don’t know
- you need a second known spell that will replace inactive talent
- just define a neutral tooltip using #showtooltip [known:ØØØØ] Your Talent; Neutral Icon
- Blabla Blablabla Bla…
« …that’s not what I was asking! » 
— I neither want some random second spell nor a random neutral icon placeholder! I want my macro’s icon to just darken /grey-out when not in use.
– – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – –
Well, I figured out a way to get it done!
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JUST BE A LITTLE CREATIVE
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it’s probably not what you expected, but hey: it works! It works exactly how you want it to, so be a bit open minded!
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STEP #1 — The Dummy Item
You need an item /toy /pet /mount /anything that, due to its specific requirements is not usable outside it’s restrictions (specific content /events /zones /faction & co.) and therefore is dimmed by default !
Some examples:
- NOT USABLE mounts due to restrictions as water-mounts or zone-bound
Red Qiraji Battle Tank • Green Qiraji Battle Tank
Yellow Qiraji Battle Tank • Blue Qiraji Battle Tank- If you NEVER play PvP
PvP toy H.E.L.P. • Horde Battle Standard • Alliance Battle Standard
Djaradin’s “Pinata” • Horde Glider Kit • Alliance Glider Kit- If you play HORDE only try any Alliance mount
Ebon-Gryphon • Snowy-Gryphon • Golden-Gryphon- If you play ALLIANCE only try any Horde mount
Blue Wind Rider • Green Wind Rider • Tawny Wind Rider
Choose ONE
link it in the chat’s edit / typing-frame without pressing enter
press Ctrl+A (select all) then Ctrl+C (copy) and hold it in memory
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STEP #2 — Get your Macro
mine is a simple Prot Pally macro to cast BoP on my focus by default, if I have one, on my target if I don’t and & on myself by holding shift.
#showtooltip
/cast [mod:shift,@player,known:1022] [@focus,help,nodead,known:1022] [@target,help,nodead,known:1022] Blessing of Protection
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STEP #3 — Set Icon
when you have your macro open, click on [ Change name / icon ]
search for the same icon of your macro’s talent, select it & save
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STEP #4 — The hidden Dummy item
now add the chosen Dummy item to tooltip after the talent’s part, like this:
#showtooltip [known:1022] Blessing of Protection; Blue Qiraji Battle Tank
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STEP #5 — Ready for Actionbars
the creative solution macro is done, drag it to you actionbar & enjoy 
Complete Macro:
#showtooltip [known:1022] Blessing of Protection; Blue Qiraji Battle Tank
/cast [mod:shift,@player,known:1022] [@focus,help,nodead,known:1022][@target,help,nodead,known:1022] Blessing of Protection