You can tell some basic things about color from the first leaves, but many seedlings change as they grow. The color yellow will be apparent immediately, but as the plant matures it may range from very pale yellow to deep gold. Greens are also very obvious from the first little leaves, and you should be able to tell within a few weeks which will be light green and which dark green. Blues are a bit more problematic. They appear green initially, but with a slight bluish cast to them. If the plants are bottom watered so as not to wash off the "bloom" that causes the blue color, they will become more and more blue as they mature. However, most blues will gain in intensity over the course of several years, and of course their level of "blueness" will vary from year to year with the weather conditions. So although you will be able to tell that you have a blue plant, you won't know the true mature color for quite some time. Variegation is almost always apparent from the first tiny leaves. The exception to this is plants with a very thin edge of white or yellow, which will not be apparent until the plant puts out some larger leaves. Two examples of this type of variegation are Gloriosa and Dew Drop. Wide light edges or light centers will be apparent very early, but again, you won't know the final appearance of the plant until it has had a few years to grow. Light edges in particular are very likely to widen dramatically as a plant matures. Patterning, as in Tattoo and Striptease, will be there in a small plant, but obviously you won't know the true mature pattern until the plant has grown for at least a year, usually more.
Streaking is apparent immediately, but of course will be extremely variable from one leaf to another. Streaked plants are also highly likely to revert to either solid colors or to take on a basic light edged pattern as they grow. These light edged varieties are often refered to as the stable forms of a streaked plant.
The color of mature variegation will not be apparent in a young plant. That is to say, you won't know if the mature plant will have white, cream, light yellow, or dark gold variegation. Many hostas in the garden emerge with dark yellow variegation which changes to cream or white over the course of the summer, and many seedlings that start out with yellow also mature to a lighter color as they grow. Even on plants that have a yellow leaf with a white edge the shade of white will often lighten as the plant matures, going from cream to bright white. The color of the variegation is also very dependent on light levels.
Red petioles may or may not be apparent in a young plant. The color red in hostas is extremely heat sensitive, fading with high temperatures. It will be at it's best in the spring, sometimes disappearing completely by mid summer, sometimes changing to a dark purplish color. Since most people grow their seedlings in a very warm environment, the red may not be visible until the plant has been moved...