Once the data is persisted in the mosaicked ECW file, there is no way to get rid of the various inconsistencies between the colors of the various tiles. You need to go back to the source data if you want to try and balance or match the colors before you mosaic them into one file.
Assuming you have the source data (and you are using ArgGIS 9.x), I would first create an UNMANAGED raster catalog and load in all the raster datasets. Make sure that statistics have been calculated on all the source files. Now in ArcMap, add in your raster catalog. Open up the Raster Catalog properties. Click on the Display tab, and choose the Never Show Wireframe option. Click on the Color Correction tab. Here, you can adjust the colors until you are happy with the result. Note that since color correction works without a contrast stretch, your image may appear dark (but that can be fixed later when we calculate statistics on the mosaic). When you are happy with you result, right click on your raster catalog, point to Data and then click Mosaic Raster Catalog. In the Mosaic Raster Catalog dialog, make sure that the Apply Color Correction box is checked, and output your data to a JP2 file with compression quality of approximately a 25. (ArcGIS does not write out ECW files).
If you have ArcGIS 10 and have the source data, you can create a mosaic dataset instead, and then color correct it.
