The color of the button is derived from your Windows "style" selection external to Access. That is, it is derived from Windows itself, from the defaults used to make new child windows. You cannot select the button color in Access unless you change the button color for ALL WINDOWS. Which I don't recall is something you can so easily do.
Having said that, you can still "cheat" by finding that button and displaying something on top of it that LOOKS like a button. Or you can replace the button with something else that you CAN control very easily.
For instance, you can make a TEXT BOX or LABEL BOX with chiseled or raised borders, put some text inside it, and have an OnClick routine. (If you use a text box, lock it.) If you REALLY wanted to get fancy, you could make the borders of the box change for a moment inside the OnClick code, then execute a RePaint, do your code, reset your borders to their original state, do another RePaint, and exit.