Have faith in your own instincts, there are absolutely no rules in bereavement, it is really important that you follow your heart and do what feels right for YOU!
No two dogs are the same, you‘re not trying to replace Ruby, you couldn‘t even if your tried, she still holds a special place in your heart and that wouldn‘t change. Love isn‘t rationed and you‘ve obviously still got more than enough to go round. Whether you decide to have another dog that looks the same is personal choice but I would think if you had any doubts about how you might feel, then go for another colour but all dogs have their strengths and weaknesses, as do people, and you‘d accept the „faults“(for want of a better word) and rejoice in the strongpoints of a new dog in the same way as you did with Ruby, I can‘t imagine any comparisons you might make would influence your feelings and love for either of them.
I did wait quite a long time between my dogs but that was more down to circumstance than anything else. A friend of mine, a dog trainer, lost her old dog recently that sh’d d had for 14 years, she went to the dog home the following day and came home with a young rescue dog. I personally couldn‘t have done that but she felt it was right, she adored the old dog and is besotted with the new one, so for her, it was the right thing to do.
I would just giive yourself a bit of time but keep your eyes open and when the right dog comes along, you‘ll be ready and will know.
I wish you all the best, be kind and patient with youself and do keep us posted!