Pan sharpening is a well documented image processing technique and you will find bunch of HowTo's and tools in the web (GDAL, ArcGIS, Orfeo TB, Grass, ENVI).
On a common data oriented processing approach using Geotiff for example, you can use open source software in 3 steps:
Synchronize the PAN and the RGB (or multichannel) stuff spatially which means, that both datasources (PAN and RGB) have to use the same coordinate system, same footprint and the same raster size. You can do his with the tools from the GDAL biotop for example (
One way to do the sharpening step is, to transform the RGB colour space of the low resolution image into the HSI colour space (aka IHS, HSV, HSB) and replace the intestity (I - channel) with the high resolution PAN data. If you stay in the GDAL biotop, the tool
hsv_merge.py can do this job (step 3 too).
At least you have transform the resulting HSI image to back to RGB.
There are different ways "to inject" the hires image into the lowres RGB to get the fused image (Brovey, PCA, etc.) with their pro's and con's. If you are familar with
GDAL library and a programming language like C or python you can find and adopt/apply programs like
gdal_landsat_pansharp especially for landsat.