The ice/inline skate shop round the back of the bus station/Premier Inn has a pretty good selection of laces, but cant remember if they had orange or purple ones, sorry!
I found some nice knee length socks in TK Maxx too. In fact Golddigga do derby style knee highs, one of which, if I'm not mistaken comes in those colours?? Might be worth a look.
In case anyone is reading this fir a 'where to buy' Sports Direct in Broadmead has football/rugby socks in loads of colours... except pink, the bloke gave me a weird look when I asked for pink! :/
In terms of length, personally my size 4 skates require at least 180cm. I guess 200 would be ideal, but 180 are the longest 'sports laces' that Timpson sell in Asda!