Mix a tiny bit of a neutral colour (so like an off-white for a light colour, or a grey) that should help. Also, you could try painting the light/dark of the model in greys, then using the colours as glazes over that.
Adding more neutral accessories and decor to the room with white, black, and gray will help absorb and neutralize the hue. You are basically toning down the color in the room. This could be done with larger pieces like furniture or smaller accessory items.
When you mix a paint color, you start out with a can of white base paint, and you drop a certain number of other colors into it. He gave us an example of a paint color being 10 drops of yellow, and 10 drops of black. To lighten any color by 50%, you would cut the number of drops in half.
When a color is too bright, you want to “gray it down.” This means neutralizing the color by adding its complementary color to any degree that you want—either on the warm side or the cool side—which means the color you make may not necessarily be gray.
Adding a little bit of a complementary color to your first color will help dull the shade down. For example, if you wanted to dull down red paint, you would add a little bit of green paint to it. The more you add of the complementary color, the more grey the first color will become.
The farther apart on the color wheel the two colors you are mixing are, the more the intensity of both color is diminished. Ultimately, mixing two colors that are completely opposite each other on the color wheel (complementary colors) creates the least intense (most neutral) mix.
To lighten your colors, just add a little water to the paint: the more you dilute it, the lighter your shades will be. For darker colors, add paint. Note: without water, the color will be more opaque and cover the previous layer of paint.
Black and white method
The more white, the lighter the tone, which is called a tint. Equal amounts of black and white create a mid tone and more black than white creates a shade. When using watercolour, the white of the paper acts as white paint, so to lighten a tone, add black and water.
A color can be made less intense by adding gray to the color (Figure 5). In some ways, intensity can be measured by the amount of gray in the hue.
There are four different ways to lower the saturation of a pure colour. You can either add grey, add white, add black, or dilute a pure colour using its complementary colour. Here you will lower the saturation of a colour by mixing it with its complementary colour.
For instance, darkening a color by adding black can cause colors such as yellows, reds and oranges to shift toward the greenish or bluish part of the spectrum. Lightening a color by adding white can cause a shift towards blue when mixed with reds and oranges (see Abney effect).
Every paint color has a formula, and you can either lighten (add white) or darken (add black) a formula to your liking. The LRV (light reflective value) of a paint color changes slightly based on how you darken or lighten it, but there's no way to tell by how much (for reference, black is 0 and bright white is 100).
Paint along the bottom edge of the color on the canvas, moving back and forth as you go. Move down the canvas a little bit with the color, using broad strokes to blend it in with the color above. As you move back and forth, it should blend into the first color, lightening a bit as you move down.
Keeping all of this in mind, there are a few ways color can be toned down while mixing without dramatically changing a color's identity. The simplest of these being additions of white, gray and black.
When a color is too bright and needs to be toned down, or you want to create a shadow, mix a small amount of the color opposite on the color wheel to “neutralize” the color. For example, add a small amount of ultramarine blue to cadmium orange to take a little of the fire out of the orange.
When you water down exterior paint, it changes the color by making it lighter. This means that the end result often differs from the sample patches or the digital renderings, if the contractor provided them at all. Watered-down paint also causes inconsistencies, especially when the water and paint are poorly mixed.
Adjust saturation and hue
Use the Saturation slider to make colors more vivid or more muted. For example, you could add a color punch to a landscape by saturating the colors in it. Or, tone down a distracting color, like a vivid red sweater in a portrait.
Compensating: If color is: Too Blue: Add small amount of Black or Brown. Add small amount of White to compensate darkening effect. Orange may also be added to neutralize Blue. Too Red: Add small amount of Green, Blue or Black.
The trick to muting a bright color is mixing in a bit of its complement. When painting with my limited color urban sketching palette, I approximate complements by picking the closest color in my palette, which works pretty well. In the diagram below, I dull down an orange ( #1 ) with a blue from my palette ( #2).