Different Colours are possible:
Red, Black, Green, Magenta, Purple, Teal, Yellow
The Spawner is one of the enemys buildings. As the name says it spawns the enemy beasts within a fixed time period. Also the amount of the spawning beasts is pre defined. Every spawner has different spawn properties and it is not defined by the colour. The colour only shows the difficulty of the spawner which has the following ranking (from easy to harder):
Some spawner are able to spawn the double amount of beasts when reaching a "special wave number" for example every fourth wave or every tenth wave. The spawner also defines the beasts level that gets increased by passing a doubled wave. If the final wave is reached some spawner will spawn a final wave with one or two strong bossbeasts and some additional beasts . If all beasts including the bossbeasts of the spawner are killed, the spawner will destroy itself and the match is won (if there is no additional spawner spawning).
Also some of the spawner raises the beast amount after the doubled waves.
The spawner can be clicked/marked with the left mouse button to get more detailled information about the upcoming beasts .
If the beasts spawn to slow and the next wave already approaches, there will be an overlapping wave that allows the spawner to throw out all beasts at once for both waves. (the beast amount stays the same)