As long as both parent plants in a cross are reasonably pure lines, the F1 progeny will be largely intermediate between them. For example, crossing a pure line yellow with a pure line red will produce an intermediate orange in the F1 progeny. All of the progeny will be very similar with each other, as long as the parents are a pure line.
Natural lines from the wild, especially where the population is small tend to be reasonably pure lines. If in doubt, the parents may be selfed (crosses made to themselves), and the seed grown to examine the purity of the line. This process may be repeated for several generations if desired to create purer lines.
Hybrid lines from a nursery are usually of mixed parentage. Breeding with them will produce varied results. Hybrid lines may be purified through selfing, as well, but if you do this you will probably find that these lines have been extensively inbred which makes further breeding difficult (the pollen and seed may become weak when the line is inbred for too many generations). This is called inbreeding depression and here is a link about it.
Once you have grown F1 progeny which show promise, you can self them to purify the line, and to look for desired qualities. Selfing of F1 progeny from two fairly pure line parents tends to produce classic Mendelian-type results. For example, if one parent is red, and one parent is white, we would expect in the *second generation* to achieve 25 per cent red, 25 per cent white, and 50 per cent intermediate (pink?). The same type of results would be expected with many of the other qualities of the plant. An exception will be the dominant, recessive, and linked qualities, which bush mimulus are known to display. These will require special attention to emphasize or de-emphasize as desired.
Some of the qualities known to be dominant in bush mimulus are non hairyness of stems and leaves, thick stems, long pedicel, excerted corolla tube, rotate or recurved corola lobes.
Some of the qualities known to be linked in bush mimulus are large flowers tend to be linked to large plants; reddish stems linked to red flowers; pale stems and leaves linked to yellow flowers.
Some of the qualities known to be intermediate or neutral in bush mimulus are flower color, leaf color, leaf shape, plant size. These qualities tend to display Mendel's results.
Other inherited factors include flower fading, width of petals, pinched or open throat, flower shape, flower petal ruffleing, floriferousness, uprightness or hanging plant.
[updated 3/15/2000]