Yes, more books are planned, but whether any and if so how many will actually be written seems to be unclear at this point.
The probably most up-to-date thing I can find from the author at the moment is a short note on his website about a prequel for next year:
2018: USA/UK Publication of Shades of Grey Prequel: 7 Things to do before you die in Talgarth
But according to this reddit thread this release date has been pushed back a couple of times from 2015 already.
Furthermore, it seems that some early editions of the book included titles for two sequels:
He plans two more books: it says in my copy that Brunswick & deMauve will return in:
- Shades of Grey 2: Painting by Numbers
- Shades of Grey 3: The Gordini Protocols
However, the release for Painting by Numbers has been pushed back several times. Originally, it was supposed to be released in 2013 according to the linked thread on SFF chronicles, but there is currently no official planned release date. Fforde explained this delay in an AMA in 2014:
I was immensely proud of 'SoG' and had high hopes for it, but initial sales were tepid, despite generous reviews, hence the lack of a sequel.
This comment also contains some information about the prequel (in spoilers, in case you don't want to know until it's out):
A prequel to 'Shades of Grey' that is essentially a standalone set in the Shades of Grey world, two weeks before the Something that Happened, referred to in Shades of Grey. Jane and Eddie won't be born for another seven hundred years, but there's still a lot of fun to be had.
Then in May 2016, Fforde replied to a comment on The Guardian saying
I like this book, but it has had disappointingly low sales - although things are picking up now. I'm planning on writing a sequel as the bokk [sic] after the book after the book I'm working on now, so maybe 2019.
Looking at the current timeline he may also have been talking about the prequel again, but there's no way to tell for sure. This is the latest update from him I can find (apart from his website), so I'm afraid you'll still have to wait it out and see whether anything will actually be released over the next years.