Usually when you plant a seed in the garden you are left down to luck and can only hope the seed grows into the colour you want it to. However there are special seeds you can obtain that guarantee a specific colour; these can help you track down those harder to get Moshlings. 

Due to their powerful nature, they aren't often available. Your best bet is to check Super Seeds to see if any are in stock. If you spot them there, they will cost a high amount so as not to encourage bulk buying. There will also be codes distributed in the future. Rest assured that with patience you will find the seeds you need to track down those pesky overlapping Moshlings like Oddie and Raffy!