Why should a blue phone have an asking price $70 higher than a yellow phone? It's a simple matter of supply and demand.
The price tag on the blue phone above says "rare", suggesting that the price differential between blue phones and yellow phones stems from differences on the supply side.
Younger readers might wonder why there would be such large variations in the number of phones made in different colours, and so few styles of phones available. Back in the 1960s, Canadian consumers were not able to buy phones. If they wanted a phone, they had to rent one from the telephone company. (Each province had just one telephone company.)
The basic rental model was a black, rotary dial phone. Many of these phones ended up being shipped to overseas after the phone company's monopoly on phone provision ended (I picked up the one shown on the left in Cuba), but they can still be found in second hand shops across the country. Any special options - coloured phones, more than one phone per household, or, in the 1970s, touch tone dialling - cost extra. It might be that few consumers were willing to pay more for a baby blue phone, or it might be that the telephone companies thought such phones would not be demanded and did not stock them. In any event, it seems that there are not many around today.
The blue phone-yellow phone price differential might also reflect differences in the demand for each colour of phone. Perhaps blue phones are now considered to be intrinsically more desirable than yellow ones. Or perhaps people, observing that blue phones have increased in price more rapidly than yellow phones in the past, see blue phones as an investment.
One final caveat: the prices shown are asking prices. They may bear little relationship to the actual market-clearing price of yellow and blue phones. Elsewhere in the flea market, a copy of a Fabian's "Hold That Tiger!" LP had an asking price of $50.
I seriously considered buying it. But I don't know if it would go with my baby-blue rotary dial telephone.