How you invoke depends on your relations to fire. Keep it simple if you are not a fire person, go all out if you are a closet pyromaniac. Use as little light as possible while casting and invoking. That will help set the mood and will make it easier to see the flames. Any colors you may see will be enhanced by the darkness.
Fire is finicky.
If the invoking flame keeps going out or the bowl just will not light, don't force it. Let it go and try again some other time. You don't want to see the visions that come from unwilling flames coerced into life with the repeated application of flammable liquids and mechanical aids. Sit comfortably and stare into the fire. Watch the flames and see what shapes they make.