• Use RenderSystem.getShaderColor() to get the current shader color before setting the new one, then use these values in the setColor() call and just change the alpha channel (because it's an alpha fix, so only alpha is important)
• FIX HERE:

  FancyMenu/common/src/main/java/de/keksuccino/fancymenu/mixin/mixins/common/client/MixinImageButton.java

  Line 38 in cfa9a65

  graphics.setColor(1.0F, 1.0F, 1.0F, ((IMixinAbstractWidget)button).getAlphaFancyMenu());

• AND FIX HERE:

  FancyMenu/common/src/main/java/de/keksuccino/fancymenu/mixin/mixins/common/client/MixinImageButton.java

  Line 48 in cfa9a65

  RenderingUtils.resetShaderColor(graphics);

Fixed/Added in FancyMenu v3.3.0!