here you go. There are no real conventions, it varies from author to author. The only ones I kind of see are blue is often used for optional objectives, and red is used for warnings - like AV warnings.
And please use colors sparingly: lighting your text up like a rainbow brite can actually make it harder to read.
Also stay away from dark green on black, it's hard to read for many against the green background behind the contact holograms.
I was not aware that so many color options were available.. or rather I thought our selection was limited to only those listed in the editor. This is fantastic! I know what I'm doing later today!
Story arcs:
The Golden Scepter: #9852 [Winner of American Legion's July 2011 AE Author Contest]
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The convention that I am used to seeing (which is not official at all, just what I'm used to) is something like this:
Bold and/or larger type for the title of a mission.
Yellow text for Significant Keywords and Phrases in a mission briefing, send-off message or mission debriefing. A player with low attention span could possibly skip over your carefully crafted writing and only read the colored text and still get the idea that they need to kill BadGuyBoss and find the MacGuffin.
Yellow or red text for AV warnings in a mission briefing or send-off message.
Light blue text in the Nav Tool for optional objectives.
You theoretically have 16 million possible colors you can use for your text, but exercise caution in using lots and lots of color in your text. I've seen some mission briefings use multiple colors very effectively, but I've seen others that are nearly unreadable due to clashing colors. Personally I'd avoid using lots of colors unless there's a good story reason for doing so.
@PW - Police Woman (50 AR/dev blaster on Liberty)
TALOS - PW war journal - alternate contact tree using MA story arcs
=VICE= "Give me Liberty, or give me debt!"
I've been trying to take some color queues from new mission content that has been recently released (the Incarnate Trials, Mortimer Kal, tip missions, etc.).
One of the big changes I've made, which in fact has forced me to delete my arc entirely for a complete overhaul, was using text color to offset action vs. dialog in a window (I currently plan to use Orange text to denote "stage directions" and leave the "dialog" in its usual text color).
I personally have been using the highlight method, where you highlight the text that you want colored, right click, and select color and the color I want. This easily and automatically insert the tags that I want. I don't have to mess around with the HTML Color Wheel and the options available are what I'm guessing the developers most often use also.
I've noticed that in my testing, that this simple change with color really spiced up the story because I am able insert sections between the original dialog where the contact supposedly pauses to think about what you have discovered in the last mission. The contact then looks at you cautiously, as if to consider whether to let you continue with the next mission. And finally I go back to the dialog that was originally written asking you to help them out.
I realize that my post doesn't specifically talk about color conventions, except that maybe I recommend running through the new mission content. But it does talk about a good use of color. I agree with Lazarus (from earlier) that color should be used sparingly. It should also be consistent as well. If you use one convention on the first mission, be sure to use it on the second and third.
Thanks all--spent the night applying colors to my to arcs--now hopefully, they'll be all shiny and attractive enough that someone will play them... 
Try out my AE Arcs! (solo friendly, canon related, & some contest winners)
| #529832 Rularularian - Heroic - 41-54 | #515140 The Serpent Beyond the Horizon - Heroic - 46-52 | #183832 Save the Diver, Save the World - Vigilante - 25-30 | #245534 Condemning Croatoa - Villainous - 25-33 | #540585 Robolution - Villainous - 25-34 |
I am realizing that my arcs show their age without color coding the mission text. Is there a style guide to color conventions as well as the tagging codes to use them?
Thanks in adavance!
Try out my AE Arcs! (solo friendly, canon related, & some contest winners)
| #529832 Rularularian - Heroic - 41-54 | #515140 The Serpent Beyond the Horizon - Heroic - 46-52 | #183832 Save the Diver, Save the World - Vigilante - 25-30 | #245534 Condemning Croatoa - Villainous - 25-33 | #540585 Robolution - Villainous - 25-34 |