I wonder if this has something to do with the ‘blendMode’ property? This can take two values: ‘avg’, which means that when colours with an opacity of 1 are overlaid they do not mix; and ‘add’, which means that colours will be blended (https://www.psychopy.org/general/rendering.html#blendmode). For the GratingStim object, ‘blendMode’ is listed as an attribute with a default value of ‘avg’, so colours will not be blended. Whereas for the ImageStim and ElementArrayStim objects, ‘blendMode’ is not listed as an attribute and it is unclear what will happen when colours are overlaid. My understanding of this is limited, but since the ‘tex’ and ‘color’ properties of visual objects both have RGB values, these objects are effectively overlaying two colours.
The documentation (https://www.psychopy.org/general/rendering.html#blendmode) seems to indicate that ‘blendMode’ generally has a default value of ‘avg’, but the behaviour of ImageStim and ElementArrayStim objects does not seem consistent with this.
See also: RGB colour space limits differ in visual.ImageStim
See also: ElementArrayStim - updating and passing lists of parameters / stimuli?