Edit: I didn't see therm's post until after I posted. I forgot to mention the obvious- quarantine definitely!
Hi Jordi, welcome to Talk Budgies. I'm very sorry for your loss.
With budgies, I've only ever kept two at a time, so I can tell you my experiences when one inevitably passed away before the other.
I'll tell you what I did with Mink when she found herself suddenly single. She did take it hard at the unexpected loss of her cage companion. She became withdrawn, moody, depressed, and made it clear she didn't want comfort from a human.
But, I did not
just go right away and get another budgie. I gave Mink some time to grieve and see if she was going to eventually adjust as a single bird. After a few weeks, I could see she was perking up and adjusting to life without her friend. Still, while she wasn't depressed any longer she seemed like she was just going through the motions and wasn't enjoying life like she used to when another budgie was around. I took into consideration that Mink has never been a single budgie in her life, and while she is tame, I can tell she much prefers birds to bond to over humans, and I don't let her interact with my parrots. Some birds would thrive as a single bird, but in this case I saw it was a good idea to get Mink a friend, not as an excuse to get another bird which is never the right reason.
I figured it might take some time to find the best match personality wise for a Mink, so after about a month after Twigs had passed, I started a search for another English budgie. Although both English and Standard are absolutely fine together, that was just what I wanted at the time. Mink is English and Twigs was a standard living together and it had nothing to do with personality. So.... as I searched for a good candidate as a new friend for Mink, I kept her sweet mild personality in mind. It did not matter to me male or female. I have the cage set up as to not encourage breeding or eggs, but that is a different topic.
You mentioned color, that is as insignificant as hair color is to humans as a factor whether we get along or not. It doesn't matter what mutation they are in the least. Age also doesn't matter. When I set out to get my new English budgie, I originally wanted some sort of eye catching unusual mutation. But that proved too hard to find in the English and especially when the first thing I looked for was potential personality compatibility for Mink. I watched the English budgies for sale at the exotic bird store. I watched them interact with each other to get a general idea of their personality. I finally ended up choosing a nice little guy. Chip, my cinnamon sky blue male in my sig. Upon observation of his interaction with the other budgies, he seemed like the best match for Mink. He is feistier and more playful than her, but they are indeed a good match and are happy together.
Observing their personality first gives you a pretty good chance of getting along, but as you know it is still never a guarantee, and a lot has to do with the willingness of the original budgie to accept a newcomer. So just be prepared to have a separate cage in case they don't get along and can't live together. When first adding a new budgie to an existing large cage, it's a good idea to change the set up as much as possible to lessen the chances of territoriality in the original bird. Best of luck