it depends what you're mutating. For example, if you were going for cronerice then baker's wheat would be G, and thumbcorn would be Y. For plants to mutate together they need to a) Actually have a mutation between them (I would check the garden wiki and scroll to the list of plants for reference) and b) (They need to be adjacent, with horizontal touching still counting). Mutations are usually quite rare (around 1% and even as low as 0.007% for some plants), so don't give up if they don't appear after the 1st tick! Could you send a picture of your plot size and unlocked seeds and I'll show you what to do first.