The design is the visual features of the DUAL COLOR AIR FILTER having a portion of variable length and width shown in solid lines in the drawings, whether those features are of shape, configuration, ornament, or pattern, or are a combination of any of these features. Shading is used in the drawings to better illustrate the design. The three-dimensional features shown on the article repeat at regular intervals throughout the length and width of the article. Color forms part of the design. Drawings of the design are included. Figure 1 is a perspective view of a front side of the dual color air filter showing green filtration media; Figure 2 is a perspective view of a backside for the air filter showing white filtration media thereof; Figure 3 is a plan view of the front side thereof; Figure 4 is a plan view of a side thereof; the other three sides being the mirror image thereof; Figure 5 is a plan view of the backside thereof; Figure 6 is a perspective view of a front side a first variant of FIG. 1 showing blue filtration media; Figure 7 is a perspective view of a backside of FIG. 6 showing white filtration media; Figure 8 is a plan view of the front side of FIG. 6; Figure 9 is a plan view of a side of FIG. 6, the other three sides being the mirror image thereof; Figure 10 is a plan view of the backside of FIG. 6; Figure 11 is a perspective view of a front side of a second variant of FIG. 1 showing red filtration media; Figure 12 is a perspective view of a backside of FIG. 10 showing white filtration media; Figure 13 is a plan view of the front side of FIG. 10; Figure 14 is a plan view of a side of FIG. 10, the other three sides being the mirror image thereof; Figure 15 is a plan view of the backside of FIG. 10; Figure 16 is a perspective view of a front side a third variant of FIG. 1 showing brown filtration media; Figure 17 is a perspective view of a backside of FIG. 16 showing white filtration media; Figure 18 is a plan view of the front side of FIG. 16; Figure 19 is a plan view of a side of FIG. 16, the other three sides being the mirror image thereof; and Figure 20 is a plan view of the backside of FIG. 16.