It depends. Water isn't. Gum Arabic isn't. Tea isn’t. Eggs aren’t
If you are using anything other than Acrylic or Watercolour or Gouache, if you are using enamels or oil based paints, then yes, though recent oil paint sticks and tubes can be thinned with water. But yes if you are using alcohol, or petroleum based thinners, then yes, they are flammable. and the fumes can be toxic.