[Rudeofus]

Experiment 2: Again you are standing on those scales but this time in a dark room. Someone a couple of metres away fires a Metz 45 flashgun at you. The scales indicate an increase in your weight and then a decrease back to your original weight. If you are a phosphorescent being (unlikely) the full return to original weight might take some hours!

Scales are stupid, they can't differentiate between force and mass. A perfect reflector will not gain mass from a flash, but the photons will exert pressure on it. The weight change indicated by the scale will depend on the direction the light hits! If you have a less than perfect reflector, some energy from the light flash will be converted to heat, which will actually increase the mass of that object until the excess heat gets radiated away. Same thing applies when a phosphorescent body takes on energy from the photons.

Experiment 3: Gold is yellow.
These experiments indicate that the light that illuminates things actually becomes part of those things.

This was trivially explained decades before photons or quantum physics were postulated. Any material absorbing/reflecting/transmitting electromagnetic waves differently dependent on wavelength will change white light to colored light.

You can make a filter that passes all visible wavelengths except for some range that gets reflected. This lossless filter will not take on any light, and the light passing through it or being reflected never becomes part of that filter. You can argue that the light and the dielectric filter layer somehow interact (else the layer wouldn't be dielectric), but we shouldn't mistake interaction for "becoming part of".