My aunt said that there was a potion to turn your pet any color, including angelic but she can't remember what it is.

Is that true or do I have to buy the potions for cash? Is there a way to change the color and species at the same time?

lets you change the color without changing the species, and includes Angelic. Though it'd be cheaper to just buy the Angelic Potion, both are available in user shops for sP.

To change the color and species at the same time, you can use elixirs

You can buy any potion with sp, some will take huge amounts but its not bad. Elixirs will always be the best bet to change a species if its not what you want in a color you're thinking about. Make sure to use the elixir BEFORE the color potion always. The highest color in elixir is blood Red, and its not available to all species. Some colors will require quests to be completed. And zapping is a total gamble, but note it will NOT apply angelic, hydrus, steamwork, or galactic.

Check out subetalodge.org for a full look at current color and species combos.

I just found this while looking for my own questions. It tells all the different ways to acquire certain colors. https://subeta.net/help/faq.php#Pets

Scroll down a bit to "What are the special pet colors and how can I get them?" and you should find good info. Keep in mind, there are always user shops and trades as well. :) I hope you find what you're looking for!