I think its good to have some obvious way of differentiating them. Especially when speaking to friends/family, who may not know much about bikes. So you can say "I'm just going out on the blue bike", then its clear which one you mean. Or even to avoid confusing yourself.
If they are different types of bikes, then that would be obvious, eg you could have a "tourer" and a "mountain bike", even if they're both the same colour. Or customise them in some way, to make an obvious difference, eg a colourful saddle or bar tape.