Images and words

I know you'll draw a sketch of mine
black and white with simple lines
and I will write a line or two
explaining and describing you
and when the sketch is halfway through
you may just add a dash of blue
and red and green will follow then
while a few more words escape my pen
and when you're done you'll realise
its somewhat true but not precise
which will make you start again
and sketch another, all in vain
and I will also come to know
that what I wrote may not be so
I'll cancel or rewrite some lines
or start afresh writing new rhymes
and we do this till you've had enough
I'll wonder instead of playing bluff
if only I saw what you sketched
and my writings to you I fetched