Things You'll Need
Measuring device (a spoon, brush, spatula or anything that lets you control and gauge amounts)
Green paint (optional but aim for a "true green" if you use it)
Mixing your own colors is a visual process, particularly when it comes to "greens" which are perceived differently by most people. Mixing the standard Olive drab color, a darker green version of olive, can be done using measuring spoons or visually if you can visually gauge quantities of paint. However you decide to mix your paint you should keep in mind that as similar as certain colors seem to your eyes, there will likely be some variations in the shades. Learning to use a color wheel will make future paint mixing simpler and will help you learn the basics of mixing colors.
Look at your color wheel and find the green color you want to make. The color wheel can help by telling you the names of which colors will create the green you want when mixed.
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Lay a dollop of yellow paint on your palette. Make sure all your paints have the same level of transparency, since mixing opaques with translucent colors tends to make a mess.
Add green paint to your yellow. If you do not have green, make green paint by mixing yellow and blue paint.
Add a shade to the basic light green you have just created by adding a bit of orange or purple. Do not use both or you will get a muddy brown. The purple will make your green a darker, more brown olive while the orange will make a more military khaki olive. Start by using a tiny amount and slowly adding more until you reach the ideal shade.
Adjust the color by adding more yellow and green if necessary. The yellow will make it lighter, while the green will make it more of a "true green" shade. Adding a tiny bit of blue will darken the green.
Adjust the hue (the level of light or shadow in a color) by adding black or white paint. Both of these shades have a strong effect so you should add it a tiny bit at a time. Black will make the color "flat" and darker while white will make the color lighter. Be careful with white because it can give colors a grayish tone if you use too much. Adjust the hues to create three dimensional and shading effects in paintings.
Mixing paint can be difficult to do even with a measuring tool so it is important to learn to gauge quantities visually (a skill that can be useful in cooking as well). If you use 10 parts yellow to 1 part blue you will get a very light green. Keep adding blue 1 part at a time until you reach your desired green (about 10 yellow to 3 blue creates a "true" green). When adding the orange and purple start with 1 part and keep adding until you reach the desired shade. Adding white or black should be done with care and you should use fractions of a part because of the intense effect these tones have.