Try this, and you'll notice the 1st data with the colon is missing. why & how to solve this?
trace("1"+":"+"2"+":"+"3"+":"+"4")
FlashDevelop's Output panel matches the first number followed by colons (ie. /^[0-9]:/) for output coloring. Different numbers will color the lines with different colors.
There is currently no way to prevent this behavior, however the information traced is still there and you can see it entirely using FlashDevelop LogViewer (see Tools menu) instead.