Oh my; where to start? I really admire the quality here, and the obvious work put into it.. that's sometimes hard to find in collaborations like these.
Probably the biggest part of this picture that draws me in is the color variety and the tones; you've mastered a wonderful range of values and, to me, that's always delightful to find. Especially with the absence of lines in the background, and the colored ones on the characters (which is a technique that I love, albeit hard to pull off at times) I think the interplay between lighting and colors is very well done.
The characterizations are also something I'd like to applaud; I appreciate the balance between caricature and realism here, particularly the expressions, which are complex but don't try too hard to capture specific features. Ughhh I'm trying so hard to type this properly and maturely or something but the coloring and their hair and those clouds and the tweed--I give up! There's just something about your art that's whimsical and thoughtful and faceted and different
that draws me in and keeps me there, looking for that one little detail I missed.
Really, if I were to nitpick anything, it'd be the lack of significant texturing on the TARDIS' body, but, honestly? The simplicity of the background against the complexity of the foreground is sort of what balances the composition out... I've got nuthin'.
Thank you for a marvelous illustration, and a good day to you!