It’s been many many years since I used one of these, but the colours indicate whether it is part of a series as defined by the broadcaster, whether it is a split programme, or a standalone programme.
Darker blue are part of a series. Mauve indicates a split programme.
Try comparing programmes that are standalone, part of a series, a split film, and it should become obvious.
Looks to me as though one colour (light blue) is your bog-standard one-off programme. Darker blue seems to be programmes with a series link. Purple maybe split programmes (ie. Films on many of the commercial channels – excluding Film4 and TPTV).
Dammit! Luke just posted virtually the same as I was posting.