The canvas gradients of HTML5 allowed me to switch from a discrete gradient to a continuous one with ease. You can set the active colour by directly entering its hex value (shorthand 3-number values are allowed) in the text field, or clicking on a colour in the mixing gradient. The latter mixes the colours with each other and with black and white. The lower-left and upper-right colours in the mixing gradient can be set to the active colour.
The active colour can be applied to elements of this ‘sandbox’.
The colour blender has been useful for myself to tune the color palette of my web site and I hope it will be useful to you as well. With proper attribution, you are free to use and modify my code under a Creative Commons licence.