Use Texturize to fill single colored areas (i.e. the bricks here) with another texture.

Use the Tolerance value to set the limit of allowed color deviation, which is still interpreted as one, single colored area.

The source pattern is supplied by the Source Texture. The offset values have no effect here, because the source pattern is shifted randomly for each area.

To have the source pattern stretched randomly for each individual texturized area, increase the Turbulence value.

The source pattern can also be variated in hue, saturation and brightness for each area.

Define with the Amplify Value how much the areas' textures should differ.

Tip: Use the Replace Mask Mode to texturize only one specific color.