I'm going to further expand this thought experiment for echoes.
Purple has the two splayers (code of laws and flute), plus some very nice echo potential on the left splay (bell / flute / novel), as well as right splay with reformation. Clock is a real killer if your opponent has a large 3 / 4 / 5 hand / score pile.
Blue has almanac / magnifying glass / telescope / ruler for some really crazy foreshadowing and drawing, not to mention a real age advancement ringer in slide rule. The magnifying glass will let you splay your blue cards left to take advantage of all these abilities. Linguistics is a pretty broken card if you have the early lightbulb advantage and a bonus on your board. Draw and foreshadow + draw is the most powerful ability. I would argue that foreshadow is stronger than meld because of the meld / execute effects.
Yellow is pretty good once you can splay right with deodorant / toothbrush, I suppose you have to wait until age 5 statistics to splay that right, you would definitely be up on leaves, and watermill leads to some interesting potential for very fast age advancement.
Red is not as useful because the abilities are kind of lame until later; however, once coke / coal / kobuksun roll around you have some interesting echoes and splay. If you count industrialization (the single most broken card in echoes) with rubber tucked, well then red takes the cake.
Green is probably the least useful in echoes, but an early umbrella / wheel is nice. However, you have no cards to splay that left until homing pigeons. Paper / umbrella tucked is VERY powerful, but you really need 3+ colors splayed to be very effective. Green also has measurement, the earliest potential instant game-ending card. That makes for an interesting wild-card possibility, but several of the green echoes are fairly poor compared to their counterparts.