I use Candle style for the price pane but especially the red sell indicator circle doesn't look very well with the red filled bar. How can I change the colors of the buy/sell indicator circle? I looked at the source code of the community component but I wasn't able to find a code.

Community.ChartStyles is what you're looking for. Grab the project's source code from our Wiki.

I searched "buy", "sell" and "color" but nothing looks relevant to the default buy/sell indicator circles. What's the keyword I should look for?

Oops, sorry - I misunderstood your request. This is not subject to alter from the ChartStyle API, it's a job of the Wealth-Lab client app and the buy/sell arrow colors aren't meant to be changed.

As a workaround, I'd suggest turning their display off with EnableTradeNotes and drawing them manually, in any color and shape, like shown in this code snippet:

WL5 Wiki FAQ | Strategies and WealthScript > Is there a way to see the trades on secondary symbols?

You can wrap all the code up in a method, pack into a compiled library, and use later in your Strategies.

Yes, that is EnableTradeNotes! I thought I have seen something like that recently but just couldn't recall where I saw it. Now figured that it was in "What's New" on WLP 5.5 user guide. Thank you!