Logic puzzle: Which octopus is telling the truth?

Source: http://math.stackexchange.com/questions/677495/logic-puzzle-which-octopus-is-telling-the-truth?newsletter=1&nlcode=97485%7cd140

King Octopus has servants with six, seven, or eight legs. The servants with seven legs always lie, but the servants with either six or eight legs always tell the truth.

One day, four servants met. The blue one says, “Altogether, we have 28 legs.” The green one says, “Altogether, we have 27 legs.” The yellow one says, “Altogether, we have 26 legs.” The red one says, “Altogether, we have 25 legs.”

What is the colour of the servant who tells the truth?

Read the discussion at: http://math.stackexchange.com/questions/677495/logic-puzzle-which-octopus-is-telling-the-truth?newsletter=1&nlcode=97485%7cd140