Hello Frances,
Try adding the second material, but set the blend mode to Multiply. If this doesn't get the effect you're looking for (I don't know the images you're using or exactly what you're wanting the secondary layer to provide), then try using the normal Mix blend mode, but set the coloring at less than 1 in order to make the 2nd material layered on the first.
Are you trying to dirty up the coloring of the object or trying to dirty up the geometry? Depending on which it is, there are completely different approaches.
By the way, this is very important to learn as you can layer up to 10 images, with each texture able to control a myriad of things... color, specularity, bump mapping, mirroring, transparency, ETC.
Here are two images... the first one shows the first Material texture applied...
Here is the same object, but with a 2nd texture applied to add additional texture interest. (ie. dirty it up)
Here is the Blendfile to examine the settings... (change Intensity of both lamps to 2 to mimic the graphic above)
Hope this helps!
James
P.S. For this one, I used both Multiply mode AND set the coloring at less than 1 (to tone the 2nd texture down a tiny bit)... Season to taste. By the way, adding the 2nd texture (trying to emulate what you seem to be attempting) made it too dark, so I increased the lighting to get the 2nd one brighter. (both lights to 2 instead of 1, only change)
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