Goopies are similar to the Digimon Pabumon in appearance as they are both goop like creatures

Pabumon for reference. 

Goopies have ovular eyes, always black. No exceptions. They come in the 14 colors of the ph scale which are

Red (common)

Red orange (common)

Orange (Uncommon)

Yellow Orange (Uncommon)

Yellow (Rare)

Yellow Green (Rare)

Green (Exclusive, meaning I can make a green one)

Teal (rare)

Blue Green (Rare)

Blue (Uncommon)

Blue Violet (Uncommon)

Violet blue (Uncommon)

Indigo (common)

Purple (Common)

As you can see, the ends of the spectrum are the most common, with the more middle ones getting rarer and rarer.  

There are no

Pink

Rainbow

Black

White

Multicolor 

Or any mary sue trait unless they get into a bucket of paint. 

Emotions

When scared or sad, they deflate into ink blots on the carpet. Thankfully its easy to clean up. 

When angry, they turn into a banana shape, though their squigly part is still on the ground. Some have claimed they look like Yellow Diamond from Steven Universe when angry.

Bits of minerals

Seeing as they are based on the periodic table, they can have bits pop out as a common mutation. These bits go along with the ph scale. Say if a goopy has gold bits, then it would correspond to the color of the goopy. 

If a Purple has no bits but they have Hydrogen arms, that's ok, your goopy is fine. That's normal since we can't see the noble gases normally. 

A very rare trait is if they have non corresponding bits.