Your issue is that you’re using smart quotes (note the slight left/right curve) rather than straight quotes ". The wikitext parser that handles things like macro parameters doesn’t recognize smart quotes, so when you type <<bigDot “r,olv,o”>>, it treats the first value as “r and the last as o”. Neither of these are in your color dictionary, of course, so they appear “empty”.
Side note: If the background color is the only styling aspect that’s intended to change, I’d recommend moving all the static CSS into a class, like this:
\function get.color(index) [[colors]getindex<index>else<index>]
\procedure bigDot(likeThis)
<style>
.dotstyle { height: 20px; width: 20px; border-radius: 50%; display: inline-block; border: 3px solid black; }
</style>
<$list filter="[<likeThis>split[,]!match[]] :map[get.color<currentTiddler>]">
<span class="dotstyle" style.background-color={{!!title}}></span>
</$list>
\end
Alternately, if you have a tiddler with the $:/tags/Stylesheet tag, you can move the contents of the <style></style> tags (but not the tags themselves) into that stylesheet to make it globally available.