While its easy to change from one thread color to another during the embroidery process its best to keep changes to a minimum. Most groups of shirts in different colors can be divided into "light" and "dark" shirts and the thread color for the text can be adjusted so it looks well on either the light or the dark group. But, the choice is up to you. Some customers have a set color for their logo that stays the same no matter what the shirt color is. Thus, depending on the shirt color, sometimes the logo contrasts well while other times the logo is very subtle.
Dark colored T-shirts cost more than lighter shirts when screen printed because dark shirts must be printed twice (increasing the cost) for the ink to stand out well against the dark shirt.
The quality of your embroidered logo can be greatly improved by finding the right embroiderer who will answer your questions and provide a quality embroidered product.
Yes, but small text and details may need to be eliminated. Your embroiderer can improve the look of your design by adding a topping during the embroidery process and slightly increasing the size of the design.