Subtitles on adverts

Something I've always wondered - why do some adverts get subtitles and others don't? Is it up to the advertiser if they want to pay for subtitles to be made?

Similarly the BBC usually subtitle their trails, but the commercial broadcasters I think don't tend to have subtitles for theirs.

It's also interesting how you tend to see colours that don't appear on subtitles on the normal programmes - for example I saw one advert using purple subtitles to match with the colour scheme of the ad, or the different background colours for SOUND EFFECTS that you don't tend to see so much on normal programmes.

Subtitles on commercials are part of the video file delivered to broadcasters, unlike programmes which usually have a separate subtitle file which plays out alongside the video and is delivered separately by the channel's access services provider. So whether or not a commercial has subs is entirely down to the advertiser.

Do adverts with subtitles cost the advertiser any more money to use them on commercial networks than they would without them? I appreciate there will most certainly be a small increase in the cost of making the advert if the decision is taken to subtitle them, but I would have thought from a bandwidth/time point of view it doesn't cost ITV, say, any more money since the "infrastructure" is already in place to do it.