Is your feature request related to a problem? Please describe.
When getting this error on a script someone else wrote it's not obvious what's happening and what the problem is without searching an explanation on the web.

Describe the solution you'd like
Adding something like

You may need to convert all images to the same color space.
pyvips.Image.colourspace function may help with that.

Additional context
Trying to just generate a text on a transparent background is unnecessarily complex and lacks an example that's easy to find.

Code fix example

                vips_image = Vips.Image.pngload(png_pathname)
                if vips_image.interpretation != 'srgb':
                    vips_image = vips_image.colourspace('srgb')