Here are some of the items and supplies that I would be lost without:
Wilton Gel Colours
I got my set from the Craft Company. The gels are great, you don't need very much to get a good strong colour.
Americolor Gel Colours (Kit #2)
Americolor Gel Colours (Electric)
Lakeland Professional Piping Set (nozzles only, the bag isn't the best quality)
I bought mine in store but the set is also available from the Lakeland website.
Joseph Joseph Nesting Bowls (2 bowls, 4 measuring cups, 1 sieve, 1 colander)

Palette Knife

PME Non-Stick Rolling Pin (for fondant, sugarpaste, regal icing)

Disposable Piping Bags

Stackable Cooling Racks

Salter Digital Scales
