Fablespinner has a great photoshop turial here at PD for doing colouring- it's how I colour. Your biggest problem your having though is colour choice since this is only flat colour.
The biggest problem is that blue. it stands out more than the subject(s) do. Tone down that blue- even though it's a flat colour. Desaturate it, or darken it. It's too bright right now. Also- the flat colours should be the natral colours of the subject- you should add washes of light or colour on top of the base colour, so that would be throwing you off too. think of it as a step by step thing.
1/ Flats- the colours everything is in natral light
2/ Identify shadows and highlights to best accent the picture's mood
3/ Add mood filters on top to push forward (accent) or push back (make unnoticable)certain areas- easiest way to know WHAT you push forward/back, as ask yourself- "What is the subject?" Depending on the mood, the subject will be bright, light, sharper, or larger than the rest of the picture. Anything else is less to not important, and should be dark, unsaturated (greyer), blurred, or smaller, depending on the picture.
There is always exceptionss of corse, but this seems to stand in most pics. That help a bit?