If a piece of jewellery is referred to as plated or electro-plated, that means that it was made out of a strong base metal (like steel, copper, or zinc alloy), and then essentially dipped it in a softer and less robust precious metal (like gold or silver) to give it a thin layer of precious metal on the outside.
Plating jewellery creates vibrant colours and protects your skin from allergies to the base metal, but the plating will eventually wear away to reveal the metal underneath.
If a piece of jewellery is referred to as colour-toned, that means that the entire piece is made out of the strong metal (like steel, copper, or zinc alloy), and the metal has been dyed to resemble another metal (usually gold).
Colour-toned jewellery generally does not contain any precious metals. Colour-toning usually produces a darker starting colour than plating, but the colour is more reliable and will last for longer.
The Cake Metaphor:
I like to use the cake metaphor to remember the difference.
If your jewellery has been plated, it's like a cake with colourful icing on the outside and plain insides.
If you jewellery has been colour-toned, it's like a cake that has been coloured using food colouring instead of icing.
Why Not Both?
After that cake metaphor, I bet you're asking the same question I was: But I can have coloured icing AND a pretty rainbow cake on the inside! Why can't I have the same in jewellery?
The answer is that you can! It's quite common for manufacturers to colour tone the base metal of their plated jewellery before they plate it, because that makes it less obvious when the plating starts to wear away. It will still fade as the precious metal wears away, but the difference is less visible.
But, most manufacturers won't tell you if they've done that, so it's kind of one of those things where you just have to hope for the best.