A frequently used costume technique is to create stark differences in opposing characters through the way that they dress. For example, you may decide to dress a dark and mysterious character in dark colors such as black, brown or gray. To complement this choice, the opposing protagonist could wear bright and vibrant colors such as yellow, red and green. This difference in character, exacerbated by costume choices, can add a great deal to each character's personality.
In the same way, these costume differences can be used to denote social status in a foreign environment. For example, a figure of power or royalty could look right at home in their environment. If the scene is composed of mostly gold embossed relics and purple furniture, the character inhabiting the room should reflect this environment. On the other hand, if a character is coming to the royalty figure for advice or a favor, their clothing could be that of poverty or dark colors. This costume difference could make the viewer feel as if the second character is alone and unfit for the environment of the first character.
Using the same thematic device to construct the costumes of similar characters can be effective in relating the characters to one another. For example, film makers often use sunglasses and suits to denote multiple mobsters or bodyguards. You may choose to use other relative devices, but these costumes must be similar enough to allow the viewer to make the similarity connection. In the same way, a similar character could have a minor difference from the rest of the group to set them out. For example, while the rest of his counterparts are wearing black sunglasses, the boss of the group could be wearing red ones. This minor difference would quickly tell the viewer that the different character in important to notice.