Solar panels are black because that means everything is absorbed. The bright colors mean much of the sunlight is reflected, decreasing the efficiency of the solar panel beneath it. Eyeballing the mural, I would guess the solar panels are about 65% as effective. The solar panels are also placed vertically, though thankfully they are perfectly south-facing, so at that latitude you can expect another multiplicative 30% decrease in efficiency. Looking on Streetview, it also appears that the eastern side of the building is offset northward, meaning it will be in the shade in the evening, so we can drop another 10% or so. Multiply all together and we get 40% efficiency compared to well-placed solar panels.

So Edmonton will need around 2.5 as many solar panels as it otherwise would have needed. This mural is 3200 square meters of panels, meaning a waste of around 1920 square meters of panel materials. According to this source, that amounts to about 3 million lost kWh per year, or about 1200 tonnes of CO2 per year assuming they are using coal to fill the shortfall, or around 100 times the per capita CO2 emissions of Canada (which is more than 100 times household emissions because industry also exists).

The fact that Guinness World Records were involved proves that it's greenwashing. They are an advertising company whose service is to make up awards that their clients qualify for to get them attention. Hiring them costs a decent chunk of pocket change.