Advice needed on repainting over a darker colour

[KarenHL]

The room is dark and gloomy, with sticker marks on the wall (don't know if it's oil or adhesive).

In the past (another house) I paid someone to paper over in lining paper, then did 2 coats of undercoat. On a tighter budget now.

The colours now are similar to:
3 walls in this blue
And one wall in sapphire salute

It was the previous owner's teen boy room - and probably gets the least light of all rooms.

I want to paint in this colour.

I've already bought this lampshade which I love.

If I use an undercoat, does anyone have any recommendations?

Otherwise I'll need to try and raise the money sell something to get it lining papered.

The previous owner of our house liked 'strong' colours, the rooms I decorated myself I undercoated in brilliant white before doing the final coat in the colour I wanted.
Some rooms took a few more coats than others - the salsa red (www.dulux.co.uk/en/products/all?id=49230&color-name=Salsa%20Red&color-rgb=AB1D2C&fl-stickey=1&color-id=1187934) kitchen was particularly painful (5 coats) as was the purple bedroom where the walls, woodwork, fireplace and ceiling were all different shades of purple.

It's worth trying a stainblock primer with a tint exact opposite of the colour wheel (from existing shade) to cut out the impact of the original colour when you intent to overpaint to paler colour, like:

1. tinted stainblock
2. ordinary primer / undercoat
3. 2 topcoats new shade