Toddlers are at a special age of exploration and are beginning to learn about simple colours. Having the same shapes painted in different colours will help a child focus on the colours rather than focusing on different shapes.
I introduced my daughter to colours at around the 24 month mark. I started using the MDH Toys Potted Gnomes with my daughter when she turned 2. Potted Gnomes were very easy to grasp and not a choking hazard. As an additional bonus, they were made of solid wood finished with non-toxic paints, so I didn't worry about her putting them in her mouth (which she did).
The first colours that were introduced were the primary colours: red, yellow and blue. We started by me saying the name of the colours to her as I handed the peg doll of that colour to her. Then I would get her to hand back the colour I asked for.
Once names of the colours were established we started to add matching colours. We put all the pots down on the table and she put the gnomes into the corresponding colour of pots. I then introduced the MDH Toys sorting bowls in primary colours and we worked with matching the mushrooms to the bowls as well (although recommended for ages 4 and up due to their size, I supervised with the small mushrooms to make sure she did not mouth these).
To help reinforce looking at colours, I also got out my paints and had her paint with the colours we were learning.
Once she knew her primary colours, we added the secondary colours: orange, green and purple. I taught her the same way with only using the specific colours we were working with and then slowly added the primary's back in to make sure she still knew them.
Things were interesting in the paint department as she was amazed to watch yellow and red paint turn orange when she mixed them. She also enjoyed watching yellow and blue turn green. But she was obsessed when she mixed blue and red to make purple. Everything from that point on had to be purple. In fact, I just gave her the red and blue paint to mix purple to her little hearts content.
We added learning pink, brown, grey, black and white - which by this point was very easy for her to remember.
She is turning 3 in a week and now has all her colours down and can tell you almost any one including light and dark colours.
I think it will be even more fun when we start learning the designer names of colours in the next couple of months. No longer will it be light purple, purple and dark purple, but if she is anything like her oldest brother was, she will start naming designer colours such as lilac, plum, eggplant by the time she is 5.