If I recall correctly, anything below Shader Model 3.0 (and sometimes Shader Model 3.0 if it gets "optimised" that way) will calculate the results of both sides of a branch and then use the results of the comparison to choose which results to keep. So, it works, and it doesn't really make things worse, but keep in mind performance-wise that you're not skipping branches. As such, it'd probably be best to change your if/else to get the four colours once and do some interpolation from there.

For example, in the code you (Carlos) posted above, you're probably sampling each texture twice.