Follow your child's lead - if he loves trains, teach the Spanish words for colors by playing with different colored trains. You can also add in some of the words that are easier, such as tren for train.
Using the play-based instruction model, you'd sit down next to her as she plays with her trains. You'd use all English, except when referring to the words in Spanish that you want to teach her. For example, since tren (trehn) is train and verde (vayr-day) is green, you'd start with holding up the train and telling her it's tren in Spanish. Ask her to then pick up a tren and have her say the Spanish word for it as she picks it up. Repeat this, working it into her natural play, until she remembers that tren in train.
Next, find a tren that's green. Tell her that the color of el tren (the train) is verde. Use similar techniques until she consistently remembers that green is verde. Then, ask her, "Can I play with the tren verde, the green train?" (Remember that, in Spanish, the adjective follows the noun. If she asks, you can tell her this but most children will just pick up on this reversal after repeated exposure.) Once she has tren verde down, move on to telling her the Spanish word for another color and, from that point on, you should be able to develop her vocabulary for all the colors.