Where possible, group your objects according to what colour they will be later, allowing you to colour them all at once and try colours more easily without having to go through and select each object individually. Select the objects using the direct selection tool, then group using Cmd/Ctrl+G. For example, if you have a field full of same-coloured trees, group them together in the layers panel.
Illustrator has some wonderful colour swatches (Window>Swatch Libraries) some of which are particularly formatted for print processes, for example Pantone Solid Uncoated which is solid (neither pastel nor metallic) colours to be used on uncoated print stock, such as art prints on watercolour or textured paper. It can be a bonus to always use Pantone swatches so you can easily get the Pantone codes if required by a printer or your client.
Don’t just choose colours that look nice. Pick colours that set the scene, create an atmosphere, depict a time of day and add depth to the scene as a whole. Here we are going for an airy, spring afternoon. Also, don’t be afraid to experiment with the outlines, either colouring them, or taking them out. Landscapes without outlines have a more authentic air to them.
When colouring your character, really take your time to get it perfect. Consider whether or not the colouring could confuse the viewer (colouring a zebra in two tones of brown may just make it look like a horse, for example). Colour the predominant areas of the character first, then colour the smaller areas and props to complement.