Yes neopets may change hit points.
no your neopet will never die
It can change color and/or type of neopet! It can also level up, get more hit points ect.
The most 'powerful' neopet is probably the neopet with the highest level and most health/hit points Majal_kita the Jetsam is the most 'powerful' neopet and its owner is featheralley this answer may have to be updated
To change the color of your Neopet.
Leveling up your neopet increases the cap of how far you can train its other stats (such as hit points or defense). At certain levels, you also unlock the ability to gain new faerie blessings.
Darmanitan will change to its Zen Mode in the events of its hit points going down to half of its hit points.
yes i have neopet on my neopet id
Yes. It's random though. So somebody could feed Glowing Jelly exclusively to their Neopet and it will never change to the glowing color.
According to neonewsnow.com the Witchy Negg 'Can change your NeoPet into a Lenny, Tuskainny, or Moehog, change NeoPets gender, boost there intelligence, raise there HP or cure any diseases the NeoPet has.'
Paintbrushes can be used to paint your Neopet the colour which the paintbrush specifies. i.e. A paintbrush is one method which you can use to change your Neopet's colour. Paintbrushes for Petpets are also available - these change your Petpet's colour.
If you mean from content to happy or something like that, just buy your neopet a cool toy to play with. :) you can obtain them from the local plushie shop in the neopet central. Some cheap toys are: pinixy keyring, red tuskaninny plushie, and blue mynci plushie.
A Nimmo is a neopet.