Hi there
I think that most old cultivars are true to seed. the ones with f1-Hybrid written on the seed package will change very much. So far I've grown Tagetes patula "Tall Scotch Prize" and Tagetes tenuifolia "Starfire Mix" from self collected seed. The striped T. patula had great flowers in the next year and the T. tenuifolia colour-Mix (yellow, orange and red flowers) was fine as well. Probably the colours will get "dull" or vary less after several generations. But you could tackle that by selecting seed only from the nicest/best plants. As for changing colour: striped cultivars like "Tall Scotch Prize" or "Mr. Majestic" can turn completly yellow because of draught and heat stress and grow bi-coloured flowers again in early autumn.