Unable to set iskeyword for colon ( : ) character

I am using vim to edit LaTeX files.

Throughout the text, there are labels to equations, tables etc. for which an accepted convention among TeX writers is the following

\label{eq:massenergy} , \label{tbl:exptresults} etc.

These can be cross-referenced later in text, eg referring to equation~\ref{eq:massenergy}, we can see that ..... blah ...blah ...

Sometimes, I want to change the equation being referenced, say I want to replace {eq:massenergy} with {eq:workenergyequivalence}. If my cursor is already somewhere in the word massenergy, I can do ciw and start typing the desired label. But in this case, it deletes the entire content within braces, i.e. becomes equivalent to ci}.

How can I set : as a word separator? Note that iskeyword+=: did not make any difference. What am I missing?

set iskeyword-=:

That is, making : explicitly not a keyword character. If : is in iskeyword it will be included in ciw.