Getting a single rainbow card would be close to the bottom of my possible options. Of course, if I can draw one or two rainbow cards from the deck, I'd be delighted, because I get two cards instead of one.
My first choice is to draw two random cards from the deck. First, there is a fractional chance of getting a rainbow card or two. Another advantage of this is that you disguise your hand. A subtle variation of this is that you increase your options, meaning that your hand is helped a lot by seemingly random cards.
Say you have the Portland to Nashville destination. Many players would begin by drawing blue cards from the board to get to Salt Lake City. That's not a good idea, because you've just telegraphed not only your route, but your destination.
Instead, your first move should be to grab the one track route from Portland to Seattle. That gives you two routes east, the blue route from Portland to Salt Lake City, and a yellow route from Seattle to Helena, meaning that both blue and yellow cards (twice as many colors) help your hand.
Incidentally, the "one router" is a "gray" route, which means that it can be taken by a card of any color and should be taken ASAP. There will be other examples elsewhere, and these gray routes represent a good use of cards (e.g. not blue and not yellow) that don't fit into your strategy. There will probably be one or two similar examples of the above elsewhere on the board, meaning that four to six colors will fit your strategies, and two to four won't, and can be used for gray routes.
If I were one card short of one or two sequences I need, then I would take the card(s) of the appropriate color(s) from the board, so that I could build the sequences the next round(s). This "draw" could be of two useful cards of different colors, or one useful card and one random card. (Again, I don't usually don't want to telegraph my intentions by taking two cards of the same color unless I "have" to.)
It's good to have a locomotive card or two in hand so that you can "rush build" a route in a race. But you will likely draw some from the deck if you draw systematically, which is why I would seldom take a single locomotive card from the board, unless I was very unlucky with my draws over many turns.