I liked the idea behind this piece, the juxtaposition of nature and the city, or nature and the man made environment. The imagery in this was lovely and I thought back to the many times I too have see the rainbow catch my eye in water on the road but I never pondered how a rainbow so beautiful exists in such a 'grimey' place as you call it.
I read Temps comments and noticed you must of taken her advice and removed the fillers because this poem is quite lovely.. I adored the use of the word 'weave' it has been a long time since I've heard it used within a poem.
I thought this was an interesting image, not only talking about a rainbow, but it reflecting within a puddle of water upon the ground and seeing the beauty within it when seeing it in a different perspective.
Only thing I can really suggest is try to remove some of your 'the' 's, there are plenty of them and sometimes you'd be surprised, but a poem will flow much better without them, they are unnecessary fillers. You may need to be a little creative with this, like in the example I'll show you below, I changed 'sun' to 'sunlight' in order to get rid of 'the'. Also, maybe add a little punctuation. I then noticed you said upon the ground and upon the concrete puddle - I don't feel you need both. One will suffice, otherwise it becomes redundant. In my suggestion I just added 'vibrantly' or something that would describe the colors and how they stand out amidst the granite color of the ground, the muck, etc. Here's my thoughts, hope it is of some help! -
chanting their happiness
upon concrete puddles
where muck and grime call home;
yet when sunlight weaves a spell,
colors dance vibrantly -
dispelling the mediocre granite tone.
You did a nice job though despite the overflow of fillers. They are quite easy to fix though, I feel this poem will be quite lovely without the majority of them. Nice job!