Exterior paint schemes can make or break the effect of a dollhouse or model building. Depending on the scale, the relationships between color or hue, tone or value and chroma or intensity of the paints chosen can accent or play down design lines. Tone or value is the lightness (or darkness) of the color. Hue is how artists refer to the differences in the colors. All of these things come into play when deciding how to paint the exterior of a model building. If you are going to be creating entire streets, you will also have to take into account which items on the street you want to draw to the viewers attention first, and which items you want to downplay.
I've used a simple three tone, or three color exterior paint scheme in the following pages to demonstrate some of the differences that occur depending on how you lay out your exterior color scheme. Experimenting with three colors in a scheme gives you a better sense of different effects than working with only two. In general these will become your body or main color, your trim color, and your accent color. If you want to progress to elaborate paint schemes, like those for the Victorian style "Painted Ladies" you may end up with paint schemes of 20-50 colors. Start with three, its easier!
I've included a pdf version (acrobat reader required) of an outline drawing of a shop front you can print off to experiment with color schemes for your own projects.
Before you begin designing painting schemes, it also helps to have a basic understanding of how we perceive colors. First, you need to understand how tones or values of color look different depending on the tone of the color they are shown against. The painting guide has a great illustration of how tones relate to other tones . The same color will appear much darker or brighter depending on what it is viewed against. The color wheel is another important aspect of understanding how to design paint schemes. Complimentary colors make each other seem brighter, and incorporating them in your color schemes can give your exterior some dramatic effects.
If choosing colors is difficult for you, you may want to learn more about color theory from About's Guide to Painting. You may also want to try an easy Hue Test, where you arrange colors like a rainbow, to see if you can distinguish and arrange similar colors.