Fine to start by choosing curtains, then you can choose new sofa covers or new sofa to match.
Take into account the size of the room and which way it faces.
Smaller rooms tend to need lighter colours, as will a room that faces north.
Darker colours are really necessary in a south facing room, which gets more sun, so colours fade easily.
If you have cats, or small sticky fingered children who visit, you might just decide on darker washable fabrics on your sofas and chairs. Ikea do a good range of furniture with loose covers that you can pop in the washing machine when necessary.
You say you have acquired more chairs over the years, do you need them all? We have opted for less clutter, which makes hoovering and dusting easier.