Just some small edits on general stuff.
The edit i made on the lamp is very rough but should give you some ideas in terms of shading.
Things cast shadows, which I show on the wastebin and on the back of the house bit. Don't be afraid of giving parts which are in the shade less detail as I did on the ground behind the house (by the bin there are no floortiles just because I could not be bothered)
On the bin you had a gradient as well, but in many cases with a super smooth gradient you do not show the form better than with lesser well chosen colours. For the little recycling icon you just made a triangle, which is recognisable but even at this low res it can be closer to what it really looks like.
In general I think it's a good idea to have outlines be shaded as well according to the direction of the light. Don't have all the outlines black, have the ones which are outlining something purple, purple and so on. This is just a general thing tho, there are no rules to this as such, esp when you are limited with colours. But in general it makes things look more organic. This I mainly demonstrated on the edited letter.
If you have specific questions to the edit, just ask.