You can play with the sprite used in this icon:
Set of white icons (ui-icons_ffffff_256x240.png)
Store this sprite somewhere, and then reference it in the stylesheet, which you can apply to the blue buttons:
.ui-icon-white { background-image: url("images/ui-icons_ffffff_256x240.png"); }
Then, when you want to set the white icon, apply this style:
<span class="ui-icon ui-icon-white ui-icon-pencil"></span>
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