I want to process a PAL or NTSC video signal and pick up colors, like green, orange, blue and red. Is there any way to decode RGB from NTSC/PAL easily, and then detect these colours? I'm looking for a small and cheap solution. The end goal is to build up an array of pixels matching a colour, then filter this to decode shapes and locate targets on a camera.
You're probably best off using a dedicated chip to do the decoding. A quick trip to Farnell's website showed a few of these under CODECs/encoders/decoders, with digital output selected. This type of chip forms the front end of frame grabbers and some displays, with chips like Brooktree bt848 recording the data to memory.
It's possible you can build a lower cost alternative using the same analog circuitry an old TV would; a comb filter to separate into Y and C components. Digital comb filters are apparently patented. Intersil AN9644 might be one document to read.