ha! i've been concentrating 3% for a couple days now for exactly this purpose! first of all, let me say, IMHO this sucks. i've got plenty of nitrate and Cr2O3 and Cr(OH)3, a blowtorch and various gases, and the hot way is superior of course as long as the fumes are vented, but i am doing this H202 method again anyways. penance for my sins.
i would not put much stock in the yellow color. takes very little chromate to give a lot of color. i say boil because with the 3%, i got nothing but this yellow color if i didn't. the solution is brought to a boil before and peroxide is added. feel free to experiment and do the opposite of everything i say though. i'm a grammar god, or so i've been led to believe, not a chemistry god.
on a side note, Na peroxide hydrate may be of good use here, i've never made it before though. this might change soon. would not have thought of this today without your questions!
no matter what concentration you use, you will need strong bottom stirring, and an addition funnel sort of device.
when the H2O2 is added, things get a little! foamy. thus the addition funnel is dropping slowly, so this does take a while. in any case, the yield with the 3% was not great, despite some variation and several tries. this may not be entirely due to the strength of the peroxide however. there is good reason why this is not a lab or industrial method. i just kept dripping H2O2 until 10-20X excess was added. i also used excess NaOH. and of course i followed no specific recipe, i resisted the internet and those library building thingies and their Cr+6 recipes with all of my Luddian strength until just a couple days before i signed up here. shows how smart/hip/wise/informed/trustworthy i am.
now the sucky part. if your solution contains ions other than K and Cr04, you have excess base, you skip crystallization because you just get yellow crap not nice pure crystalline chromate no matter how many times you recrystallize, you use too much crap-grade H2SO4, tap water, and then you concentrate and add to sat'd KCl and get? crap! crap in-crap out! how best to separate at this point? add lotsa H2SO4 or reduce back to +3! so do it right because although addition compounds of Cr+6 aren't mentioned in many books, they sure do exist! very old test for +6: a tiny bit of very diluted not-warm acid sol'n added to H202 gives a nice blue perchromic acid. very sensitive (1:40000) thus the dilution.
what i'm trying to say is that chromate/dichromate/chloride/sulfate/chromic acid doesn't always crystallize well under less than careful conditions, and even when they do, they don't do it completely. following explicit recipes from the old days or doing enough experimentation to write your own is the way to go.
i haven't been poisoned yet. the powdering and transfers of it etc. are the worst part, i think. though the dichromate can have a darkish artificial-orange-juice look to it, i never drank any. how long do you think it would last as +6 if you flushed it down the toilet? seriously. if you have a septic tank this would be a bad idea as your microbes would die, but otherwise i think not so bad in small quantity. i believe that where +6 is found in the wild, it is associated with releases into rivers or high mineral/Mn02-low organic soils. it is definitely easy to reduce in the lab. it is also easy to recycle, well, as easy as this is. in the end, it is cheaper to just buy dichromate. but this isn't about what can be bought for how much. but thanks everyone for being so freaked out over reactive chemicals. drives down the price!