If I did my math right I see
sources:
12 blue
11 red
11 white
11 green
10 black
You've indicated you need white and green early for your 3 drops. 6 of the sources share the white and green (Seaside & Jungle) so those are either one or the other but not both at the time you need to cast them. You then have to hit a second green/white and either another white, blue or red for a body to hit the board (I'd assume you stall with casting Anger to get value out of the card so lets exclude the double red). So I'm going to assume for your 3 drops that the frequency in which you cast Brimaz in the early turns is low and that you're most often getting Rhox or Thoctar out? (considering Rhox could also has the same white/ blue source issue with Sanctum and Seaside and Thoctar with Jungle and Savage for red/green)?
To be honest I'm not entirely sure what to suggest (I just haven't found any of the 5 color decks consistent and gave up on making them).
Green has the best chances for fixing through
Satyr Wayfinder,
Cultivate and
Sylvan Bounty as well as the old
Elvish Visionary cantrip (and to a lesser extent
Masked Admirers due to the cost and double green).
Blue has a few cantrips in
Fleeting Distraction,
Quicken,
Courier's Capsule,
Think Twice (which you have),
Inspiration,
Bident of Thassa (which is obviously cart before the horse),
Ior Ruin Expedition (I know),
Scroll Thief (I know I know!) and then
Traumatic Visions to fix (I'm trying to keep the cost 4 and below).
Colorless is unfortunately the best option since it won't matter what colors you have in play but your options are bleh (
Traveler's Amulet,
Armillary Sphere,
Darksteel Ingot which you have and
Meteorite which would be too late. There is also the
Avarice Amulet for draw)
If you are finding that you have enough success/ consistency that you get a stabilizer out on T4, then I'm not sure what more you can ask for from the rainbow (Maybe that can be the next commercial. Stabilize the rainbow! Taste the rainbow!).
elk