Crap in radiator

Hey up.
Just done an oil change and noticed in the radiator reservoir the blue Silkolene radiator coolant I put in about 500miles ago is brown and had a fair amount of sediment in it. I just emptied the reservoir and refilled with fresh coolant. Bit baffled by this as when I got the bike I stripped the whole system, fitted new thermostat, cleaned out the pipes etc as the radiator coolant was brown and mucky looking. Has anyone got any thoughts on this? am I being pedantic and it's just some excess sediment left in the rads or could it be something a bit more serious!
Just done an oil change and noticed in the radiator reservoir the blue Silkolene radiator coolant I put in about 500miles ago is brown and had a fair amount of sediment in it. I just emptied the reservoir and refilled with fresh coolant. Bit baffled by this as when I got the bike I stripped the whole system, fitted new thermostat, cleaned out the pipes etc as the radiator coolant was brown and mucky looking. Has anyone got any thoughts on this? am I being pedantic and it's just some excess sediment left in the rads or could it be something a bit more serious!