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My bonsais are about 2 months old now. The one silk mimosa is growing really tall. I don’t know if I must leave it growing, or trim the tall stems?
The tall one’s leaves are getting quite dark too. Is this normal? I have included a pic of the smaller ones (one is a flame tree) for colour comparison.
I have grown them from seeds.

You can let it grow for fastest trunk thickening. The con of that is you need to make bigger cuts in the future which take long time to heal.

Or you can shose to cut now, regrow, cut again and so on. This method takes painfully long time to build a mature tree but will leave you with a better quality tree in the future.

Another method is to keep the future tree small but leave sacrificial branches to help thickening the trunk. Pro is that it gives better taper in the trunk. Development time is somewhat between the two other options, but you will still leave big scars on the tree. Make sure to grow sacrificial branches on the intended back side of the tree.

Thanks for the reply.
Is now the time to start trimming it or should it be older and maybe a different season?
I am in the UK. They were indoors up until 2 weeks ago now outside with the good weather.

Weather it is to early to start pruning or not depends of your plan for the tree and your method to achieve that plan. In any case it is nothing urgent. So you have plenty of time to make up your plan.

I have no experience whatsoever with tropical trees for bonsai. But according to many experienced growers here, most tropicals can be pruned in summer.