Is there a way to make a value different in color when the default value is changed?

Right now it shows in bold, but sometimes I find it difficult to distinguish between the two font weights. For my eyesight it would be quicker and easier if it was also a slightly different color.

I don't think color can be overridden. However, you could control the font type and size. Copy the file:
$HFS/houdini/config/resources.std
to the file:
~/houdini15.0/config/resources

That file contains a list of fonts that Houdini uses. The fonts that you'd want to change are InputFont and OffDefaultParmFont, which are the font for default and non-default values. Maybe you can try changing the font size.

ありがとうアンドリューさん!

This worked well enough for me to distinguish between the two states comfortably.