What a jolly piece
Starting with a silhouette or a line drawing means that you can quickly get a feel for the entire composition of a piece, which could mean a lot less work later on should you want to change anything about it. It will also help with the shading because you'll be able to tell how the different elements interact (e.g. you might draw a lovely cactus leaf – according to Google that's a 'cladode'; who knew? – and have to re-do it if it ends up being in the shadow of another one that's added later).
When it comes to the colours, the first thing I noticed was how many of them are in the piece. You've got 43, many of which are very similar to each other. When choosing a colour to lay down, it's worth looking over the ones you've already used to see if one can be repurposed. Better yet, have an idea of the colours you need in advance and try to stick to them, only adding ones when you absolutely can't do without them.
This part of Cure's tutorial over at Pixel Joint is about colour, and is very useful, but scroll up to the top to have a look at the entire thing as well because it's very helpful. I'm not sure how small you could get your palette, but I'd advise perusing the
gallery at Pixel Joint and searching for 'cactus'. The examples there make me think that you could get to around twelve colours.
As a very quick example (and this isn't intended to be better; merely illustrative), I've stripped out twenty shades from your palette that were similar to other ones, which still leaves 23. The loss of the yellowish-green is felt, but I think you could easily rectify that by removing one of the pinks and replacing it with a bolder yellowy-green.
It also might be worth hosting your images somewhere else such as Imgur as this method means we can only see them zoomed in (hence the size of my edit).
Hopefully this is useful!
