Your file's colour count seems to be artificially inflated - it seems like as the pistons rise and recede, the whole 'screen' experiences a darkened colour tint. This might be an issue from the document you made the file in, or an issue from saving the file if it had an excessive colour count that your software tried to compress to a lower number. Either way it would be better if you edited this piece further so there weren't as many incidental/accidental excess colours.
You've obviously put some effort into this piece. You'll have a nice image for your gallery if you rectify the issues mentioned by StevenM.