Provided are some options you might like to explore.
Because of the size of the sprites you might want to stick with the flat colours. However you can give these sprites a little depth with some speculars whilst keeping the colour counts low. Depending on the backgrounds you are hoping to create, some AA might be nice on these little guys.
A couple of ways I remove colours
For the bomb character remove the yellow and opt for a white
Use some sub pixel around the right ear with some blue tones to create a purplish sense, whilst exchanging the lighter pink with the red tone.
I think the most of my changes are self evident so I will leave it at that.