Polarn O Pyret is fantastic. Bit pricier than high street but washes really well and generously sized so a 2-3 top really does last mine from age 2 to almost 4. DD is now on DS's PoP handmedowns, and DC3 (due imminently) is getting them too as they're still in good nick. I have some tops that have done 4 years so far and I expect another 2 given they're still washing so well, which is brilliant value for £15 up front.
One example we have is a striped t shirt with blue, pink, yellow and green stripes - perfect for either gender. And their patterned stuff is awesome.
If you have a girl they do lovely dresses and smocks with interesting colour combos and patterns so you can do something girly but quirky.
On the high street, I think I like Next the best for finding interesting things amidst the blue and pink. DD wears some of DS's plain long and short sleeved t shirts from there - we have red, green, yellow, orange, navy and brown from them. I was in yesterday and they have a 4 pack of t shirts in the boys section which were white, yellow, green and orange.
M&Co worth a glance too I find although that varies by season.