From Webkinz Insider Wiki
Zummies are attained in the Zums Sighting section and separately in the Visit Zums section of Zumwhere. Zummies acquired in the "Visit Zums" section can be used to purchase items in the Z-Shop. Zummies attained in the "Zums Sighting" section are used to purchase items in the Zumthings Catalog. The two types of Zummies are accumulated independently and are not interchangeable. 
Z-Shop Zummies (Yellow, Pink, Turquoise, Purple)
To obtain Zummies to spend in the Z-Shop, you must have purchased a plush or virtual Zumbuddy. You earn these Zummies by babysitting your Zums. Z-shop Zummies are light blue, purple, yellow, and pink. They can be used to purchase Furniture and Decorations for your Zumbuddy's house. They can also be used to purchase prizes from the Z-Shop that can be used in your regular Webkinz World rooms, and that are Kinzpostable and tradeable. In the lower left-hand corner of Zum room, you can see how many Z-Shop Zummies that you have.
Zumthings Catalog Zummies (Red, Orange, Green, Blue)
To obtain Zummies to spend in the Zumthings Catalog, you must purchase a KinzClip, and search for Zums in the Zums Sighting section of Zumwhere. Each time you sight a Zum and take a picture of it, you will receive three randomly-colored Zummies. Zumthings Catalog Zummmies are green, red, blue, and Orange.
You can use these Zummies to purchase items in the Zumthings Catalog found in your Zums Field Guide. The items purchased from the Zums Catalog are played with in your regular Webkinz World rooms and are Kinzpostable and tradeable.
Zumthings Catalog Zummies (Red, Orange, Green, Blue) are Capped at 100 each
If you have collected 100 of the red, orange, green, or blue Zummies, you will not be able to collect any additional Zummies of that specific color until you have spent some Zummies of that color. In other words, it is advantageous to spend your red, orange, green or blue Zummies as soon as you have acquired 100 of a given color. The good news is that you will again be able to accumulate Zummies once you have fewer than 100.
In contrast, there are no caps on the numbers of Z-shop Zummies (light blue, purple, yellow, and pink) that you acquire in the Visit Zums area of Zumwhere.
You can see how many Zumthings Catalog Zummies you have by opening your Zums Field Guide. In the picture shown, the member has 4 out of 100 green Zummies, 6 out of 100 Zummies, and so on.
To trade your Zummies for items, open the Zumthings Catalog.
- ↑ Zummies earned in the Visit Zums section ("Z-shop Zummies") can not be used to purchase items in the Zumthings Catalog, nor can Zummies earned in the Zums Sighting section ("Zumthings Catalog Zummies") be used to purchase items in the Z-Shop. Furthermore, the number of Z-Shop Zummies is independent of the number of Zumthings Catalog Zummies that your pet will have.