Ignore the following if you did mean the font itself and not the image.
It looks like someone years back had a similar problem and posted the query on another site and got an answer ..
https://www.mmo-champion.com/threads...-on-mouse-over
This is the actual line of code to change the color of the image .. set these to 0,0,0 for black
button3:GetNormalTexture():SetVertexColor(r, g, b);
What you changed in your code to 0,0,0 is the font text and not the image itself. So you may want to revert that back to what it was so that you can see the writing properly.
Give that a go and let us know how you eventually got it working ( if you did that is )