I have a paper with colour diagrams that I wouldn't like to convert to grayscale for publication. I think colour greatly helps to understand some complicated patterns. I know most journals accept colour pictures upon payment, but I'm not willing to pay for letting others publish my own work and get benefit out of it. Forum Mathematicum is a nice journal which publishes colour pictures for free, but I recently published a paper there and wouldn't like to repeat. Do you know of any other nice options? The paper is on algebraic topology bordering with quantum algebra.
I would recommend you to publish on
arXiv.org (http://arxiv.org). It's a very solid online publication outlet, free, open access and especially popular in mathematics and other hard science disciplines. Liberal guidelines with no mentioning of restrictions in terms of color.