You're in luck! It is traditional to distinguish the maid of honor from the bridesmaids by giving her a different dress or bouquet. The only question you need to tackle is what sort of dress will look best with your wedding party.
There are three ways you can put the maid of honor in a "different" dress.
Let's look at the pros and cons of each approach.
If the maid of honor looks good in the same style as the bridesmaids, this is a very simple way to coordinate the party. Use the bridal shop's fabric samples to find a color that coordinates with the bridesmaids' color. In general, soft pastels go with other soft pastels, bright light colors go with other bright light colors, and deep colors go with other deep colors. It's sometimes possible to coordinate a dark and a light shade of the same color (periwinkle blue and deep violet-blue, for instance), but this is much more dependent on what the particular manufacturer has in stock.
The potential downside is that your bridal shop may not be able to order the desired style in time. If you don't mind trying other shops, you may be able to get the dress more quickly elsewhere or buy another store's sample.
This approach allows you to choose a completely different dress that may be more readily available. Do check: if the dress is from the same maker, the order time may be the same.
Aim for a style that is similar to the style of the bridesmaids' dresses. If the bridesmaids are wearing long, slim dresses, the maid of honor's dress should be long and slim-or possibly an A-line to cover larger hips-not a gathered ball gown style or a tea-length style. If the bridesmaids' dresses are in the matte "crepe" fabric that is very popular now, the maid of honor's dress should include the same fabric and definitely should not be shiny taffeta! If the bridesmaids' dresses are sleeveless, the maid of honor's dress should be sleeveless or have very plain sleeves.
The potential downside comes with trying to match a color fairly exactly. Sometimes a color is so striking and so popular that every manufacturer seems to use the identical shade. Other times, a color will be offered by only one manufacturer. For instance, some of Watters & Watters most stunning colors are virtually impossible to match from another manufacturer.
This approach requires some skill in choosing a dress, but it frees you from the entire matching and ordering process. If your remaining time is very short, this plan has the advantage of allowing your maid of honor to buy her dress off the rack at a department store instead of waiting for an order.
How do you make this work? First, the dress should be similar in style to the bridesmaids' dresses, just as if you were choosing a similar dress in the same color as the bridesmaids. If the dress is nearly identical (same shape, same neckline, same sleeves, different back detailing), that's fine too.
Second, the dress should be a color that coordinates with your wedding color scheme but is clearly not an attempt to "match" the bridesmaids' dresses. If your color scheme is peach and green, and the bridesmaids will wear pale peach, then the maid of honor's choices are green or a dark peach that is distinct from the bridesmaids' pale peach. Use your floral choices to pull the different colors together.