a Gel filter over a speedlight would indeed accomplish that, assuming youre shooting in daylight, you would put a CTO filter on the flash, key the white balance to Incandesent/Tungsten (or the specific *K value listed on the filter), use it to light the subject and the anything illuminated by the sunlight will go blue, best use is in the shade where you dont have your speedlight figting the sun for superiority. . . you can use different strengths of filters to control just how blue you want it to go, for example if you key for a Full CTO the daylight goes really blue, if you use say a 3/4 or 1/2 CTO and adjust White Balance accordingly, the degree of the color shift will be lessened as you go down the scale
NOW that was if you wanted the subject in 'normal' color and the rest in a blue cast. . . if you just want everything in a blue cast, you can cut out the Gel filters, and even the speedlight (you can still use it as a fill light or whatever), just manually set the White Balance to Incandecent/Tungsten and everything goes blue, you can fine tune it with the manual *K settings though, daylight is right around 5500K, daylight gets bluer as the WB value decreases (ie. 5000K, 4000K, etc)
as for green casts. . .same sort of rules apply, though in this case youre looking for Fluoresent lights (particularly the 'cool white' variety). . . and this time the white balance setting would be Daylight or Flash. . . and again you can use a flash (this time naked) to get them in a 'normal' color while everything goes green, or do it ambient with one of the previously mentioned settings and everything goes green, in addition to that last one you can also use a 'corrected' flash with a Plusgreen filter as a fill light, if your WB is still keyed for daylight or standard flash, itll go green as well
Last edited by Shoji-Aoyama : 06-24-2012 at 11:50 PM.