Is the card from a set in the 8th Edition - Fifth Dawn range? If yes then this is not a misprint.
(if no, then that is an interesting find indeed...)
This was when the Magic card frame was changed and it was decided to print "mana symbols in running text in black-and-white instead of color" for a number of reasons. This decision has since been reversed.
This question was asked and answered on Ask Wizards in May of 2003. See the May 20, 2003 question from Philip, answered by Randy Buehler.
May 20, 2003
Q: "I saw one of the Two-Headed Dragon promo cards that was given away
at E3 last week. What's up with the red mana symbol in the text box?
It looks black or gray instead of red."
A: From Randy Buehler, Director of Magic R&D:
"You're right -- the red
mana symbol that appears in the 'firebreathing' ability in the text
box on Two-Headed Dragon is black text, and it will be the same on the
actual Eighth Edition cards. "We looked at all aspects of our printing
process when we were putting together the new card frames and it
turned out to be much simpler if we did the mana symbols in running
text in black-and-white instead of color. We've had problems with them
in the past, especially on foil cards, and by changing them to black
text we will be able to minimize these mistakes. R&D was initially
concerned that they would be harder to read and gameplay would be
affected by this change, so we had some playtest cards printed up and
tested with them in our Future Future League. After playing with them,
we realized that you never really notice the difference. In fact,
since the text box itself is colored, it tricks your eye into seeing
the symbol as colored too. Based on this playtesting, we concluded
that the advantages of making our printing process more efficient and
subject to fewer errors outweighed the drawback of making the cards
slightly less pretty and so we made the change. (Note that only mana
symbols that appear in text in text boxes are affected by this change
-- mana costs in the upper right and basic lands will stay the same.)"